The question in this patent case is whether the patent in suit is invalid for double patenting. The district court held on summary judgment that an earlier patent, which stemmed from the same application as the patent in suit, could not be used as a reference against the patent in suit for double patenting purposes. Bristol-Myers Squibb Co. v. Pharmachemie, B.V., No. 01-375KMLC) (D.N.J. July 29, 2002). Because we disagree with a key conclusion on which the district court’s summary judgment was based, we vacate the district court’s judgment and remand the case to the district court for further proceedings.
I
A
Research Corporation Technologies, Inc., is the owner of U.S. Patent No. 4,657,927 (“the '927 patent”), and Bristol-*1345Myers Squibb Co. is the exclusive licensee under that patent. The patent claims (1) methods for treating malignant tumors with certain platinum coordination compounds and (2) compositions containing those compounds in amounts sufficient to cause regression of those tumors. Appellant Pharmachemie, B.V., filed an Abbreviated New Drug Application (“ANDA”) with the Food and Drug Administration, seeking FDA approval to market a cancer-treating drug covered by the '927 patent. Research Corporation Technologies, Inc., and Bristol-Myers Squibb Co. (collectively “Bristol-Myers”) brought suit charging Pharmachemie with patent infringement under 35 U.S.C. § 271(e)(2). As a defense, Pharmachemie asserted that the '927 patent was invalid for obviousness-type double patenting over U.S. Patent No. 4,140,-707 (“the '707 patent”), which was issued in 1979 and expired in 1998.
B
The double patenting issue in this case turns on whether Bristol-Myers is entitled to invoke section 121 of the Patent Act, 35 U.S.C. § 121, as a defense against the claim of double patenting. That issue in turn depends on an interpretation of the prosecution history of the '707 and '927 patents.
The '927 patent can be traced to an application filed with the Patent and Trademark Office in 1972. That application, Serial No. 260,989 (“the '989 application”), disclosed and claimed compounds corresponding generally to the compounds that were ultimately claimed in the '707 patent. In addition, the '989 application claimed methods of treatment and compositions corresponding to the claims that were ultimately included in the '927 patent.
In the course of the prosecution of the '989 application, the examiners imposed two restriction requirements. The first, imposed in 1973, required that the applicants elect either the compound claims, classified in art class 260, or the method of treatment and composition claims, classified in art class 424. In .addition, the 1973 restriction, requirement directed the applicants to elect “a single disclosed species for examination on the merits.” As a result of. the 1973 restriction requirement, the applicants elected the compound claims and withdrew the non-elected method of use. and composition claims from further consideration at that time. The examiner then rejected the elected compound claims on the basis of lack of utility.
In 1974, a different examiner issued a second restriction requirement on the '989 application.. That restriction requirement identified four different compound groups within the compounds claimed in the application as, constituting independent and distinct inventions. The four groups were: (1) “Organometallic platinum compound[s] classified in class 260, subclass 429”; (2) “Platinum compounds containing ‘hetero-cyclic amines’ or [’Jheterocyclic substitu-ents’ classified in class 260, subclass 270R and many various ■ subclasses”; (3) “Compounds of the above type with 2-valent platinum and no L moiety”; and (4) “Compounds with 4-valent platinum containing various ‘anionic’ ligands.” In addition, the examiner expressly stated that the 1973 restriction requirement segregating the compound' claims from the method of use and composition claims was maintained. The applicants did not file a divisional application in response to either of the restriction requirements, but instead appealed the final rejection of the claims to the PTO Board of Appeals.
In 1977, while that appeal was pending, the applicants filed a continuation applica*1346tion, Serial No. 778,955 (“the '955 application”), and abandoned the '989 application. The '955 application presented all of the original claims of the '989 application for examination. A new examiner examined the '955 application “for restriction only” and imposed a new restriction requirement. The 1977 restriction requirement differed from the 1973 and 1974 requirements that had been imposed in connection with the '989 application. The 1977 restriction requirement mandated that the claims be separated into four groups, but unlike the 1973 restriction requirement, it did not segregate the compound claims from the method of use and composition claims. Instead, the first two of the four groups set forth in the restriction requirement referred to art groups that included methods of use and compositions as well as compounds. The first group consisted of “[o]rganometallic platinum eompound[s] classified in class 260, subclass 429 [compounds] and class 424, subclass 287 [methods of use and compositions].” The second group consisted of “[p]latinum compounds containing ‘heterocyclic amines’ or [’]heter-ocyclic substituents’ classified in Class 260, subclasses 270R and many various subclasses [compounds], and Class 424 subclass 245 [compositions and methods of use].” The third group set forth in the 1977 restriction requirement consisted of “[compounds of the above type with 2-valent platinum and no L moiety.” The fourth group consisted of “[c]ompounds with 4-valent platinum containing various ‘anionic’ ligands.”
The applicants responded to the 1977 restriction requirement by electing four claims, which corresponded to the claims that were ultimately included in the '707 patent that issued two years later. Before that patent issued, however, the applicants filed a divisional application, Serial No. 902,706 (“the '706 divisional application”). After a preliminary amendment, the '706 divisional application included 16 claims, denominated claims 5-20. Claims 5-13 were cancelled shortly thereafter. The remaining claims, in slightly rewritten form, claimed the non-elected compound groups and the methods of use and compositions originally claimed in both the '989 and the '955 applications. Following the filing of the '706 divisional application, the '707 patent issued, containing the four compound claims that had been elected from the '955 application.
The examiner issued a restriction requirement with respect to the '706 divisional application. The office action began with the statement “Restriction has been required ... between the following inventions,” after which the examiner divided the claims into three groups: claim 14, “which is drawn to Platinum (II) complexes classified in Class 260, subclass 270R”; claim 15, “which is directed to platinum (IV) complexes classified in Class 260, subclass 429R”; and claims 16-20, “which are drafted to composition and method [sic] classified in Class 424, subclass 245, 287.” In the same office action, the examiner then set forth a second, four-way restriction requirement, which replicated the four-way restriction requirement that had earlier been imposed on the claims of the '955 application. The applicants responded to that office action by asserting that the two restriction requirements seemed to be “somewhat in conflict” in that “any invention elected in accordance with the requirements [of the first] would necessarily involve election of one or more of the groups set forth [in the second].” In an effort to comply with the requirements, however, the applicants elected claim 14 of the '706 divisional application.
In 1983, after further unsuccessful appellate proceedings, the applicants filed *1347another divisional application, which again consisted of the original 1972 application. In preliminary amendments, the applicants canceled the 13 original claims and added, as claims 14-19, the claims that had been claims 15-20 of the '706 divisional application. Another examiner was assigned to the application and another restriction requirement was issued. This time, the examiner divided the claims into two groups, one consisting of claim 14, “drawn to platinum IV complexes, classified in Class 260, subclass 2S9E,” and the other consisting of claims 15-19, “drawn to methods of use and compositions, classified in Class 424, subclass 245.” In 1987, that application matured into the '927 patent. The four claims of the '927 patent corresponded generally to four of the method of use and composition claims of the 1983 divisional application.
C
The district court noted that the question whether section 121 of the Patent Act is available to Bristol-Myers depends on whether the applicants were required by a restriction requirement to prosecute the claims that ultimately became part of the '927 patent separately from the claims that became part of the '707 patent. The court concluded that the statutory requirement was satisfied because “it is evident that the original 1973 restriction requirement remained in effect and required the applicants to pursue their method of treatment and pharmaceutical composition claims in a divisional application. This restriction requirement was never cancelled, revoked, or withdrawn.” Accordingly, the court concluded,
the divisional application pursuing method of treatment and pharmaceutical composition claims was filed as a result of the restriction requirement and was not a “voluntary” act; over the years, the applicants made repeated attempts to traverse the PTO’s restriction requirement but were not permitted to combine compound claims with method of treatment and composition claims.
Because the court concluded that section 121 barred the assertion of double patenting as a basis for Pharmachemie to assert the invalidity of the '927 patent, and because Pharmachemie abandoned any other defense against Bristol-Myers’ claim of infringement, the court entered final judgment of infringement. Pharmachemie appealed.
II
Section 121 of the Patent Act provides, in pertinent part, as follows:
If two or more independent and distinct inventions are claimed in one application, the Director may require the apr plication to be restricted to one of the inventions. If the other invention is made the subject of a divisional application which complies with the requirements of section 120 [of the Patent Act] it shall be entitled to the benefit of the filing date of the original application. A patent issuing on an application with respect to which a requirement for restriction under this section has been made, or an application filed as a result of such a requirement, shall not be used as a reference either in the Patent and Trademark Office or in the courts against a divisional application or against the original application or any patent issued on either of them, if the divisional application is filed before the issuance of the patent on the other application.
35 U.S.C. § 121.
As section 121 has been interpreted by this court, Bristol-Myers is entitled to *1348invoke the statutory prohibition against the use of the '707 patent “as a reference” against the divisional application that resulted in the '927 patent only if the divisional application was filed as a result of a restriction requirement and is consonant with that restriction requirement. See Geneva Pharms., Inc. v. Glaxosmithkline Pic, 349 F.3d 1373, 1378, 1381 (Fed.Cir.2003); Gerber Garment Tech., Inc. v. Lectra Sys., Inc., 916 F.2d 683, 687 (Fed.Cir.1990). The district court held that the divisional application that led to the '927 patent was filed as a result of, and was consistent with, the restriction requirement issued in 1973. According to the court, that 1973 restriction requirement resulted in the 1978 divisional application that ultimately resulted in the '927 patent, because the 1973 restriction requirement “remained in effect and required the applicants to pursue their method of treatment and pharmaceutical composition claims in a divisional application.” Although the 1973 restriction requirement was issued against the '989 application, and not against the '955 application, from which the 1978 divisional was filed, the court ruled that the 1973 restriction requirement applied to the later application because it “was never cancelled, revoked, or withdrawn.”
Our review of the district court’s summary judgment order in this factually complex case presents a relatively straightforward question: whether the district court was correct to conclude, as a matter of law, that the 1973 restriction requirement was applicable to the 1977 application and therefore resulted in the 1978 divisional application.1 The district court held that it was and that the patent therefore cannot be cited as a reference against the '927 patent for double patenting purposes. Pharmachemie, on the other hand, argues that the 1973 restriction requirement was not in effect at the time of the filing of the divisional application that matured into the '927 patent, and that the '927 patent therefore cannot be said to have been filed as a result of that restriction requirement.
We agree with Pharmachemie. The '955 continuation application, which was filed in 1977, began a new proceeding in which all of the original claims of the '989 application were once again presented for examination.2 In 1977, when the examiner for the '955 application issued the restriction requirement for that application, she did not reinstate or even advert *1349to the 1973 restriction requirement. In fact, the 1977 restriction requirement that she issued at the outset of the prosecution of the '955 application was different from, and inconsistent with, the 1973 restriction requirement. The 1977 restriction requirement, unlike the 1973 restriction requirement, grouped compounds together with methods of use and compositions in at least two of the four invention groups, while the 1973 restriction requirement directed that compounds be segregated from methods of use and compositions. Moreover, the examiner examined the method of use and composition claims “for subject matter of [the elected groups] readable on the elected species” as reflected in the subsequent office action. This suggests that the applicant could have complied with the 1977 restriction requirement in a way that would have been contrary to the categories set forth in the 1973 restriction requirement. By imposition of a new and different restriction requirement and failing to make any reference to the restriction requirements imposed in connection with the parent application, the examiner made clear that the previous restriction requirements did not carry over to the '955 application.
Bristol-Myers argues that the examiner in effect adopted the 1973 restriction requirement in the course of the prosecution of the '955 application. Bristol-Myers suggests that the four-way restriction requirement of 1977 incorporated the two-way restriction requirement of 1973 and thus resulted in a six- or eight-way restriction requirement, part explicit and part implicit. There is no indication in the record, however, that the PTO intended one of the two restriction, requirements imposed on the '989 application to carry forward to the '955 application, but not the other. Moreover, the record does not indicate that the applicant proceeded under the assumption that the 1973 restriction requirement continued in effect. During prosecution of the '706 divisional application, when a restriction requirement similar to the 1973 requirement appeared in conjunction with a restriction requirement similar to the 1977 restriction requirement, the applicant noted that the two requirements were “somewhat in conflict” and that “any invention elected in accordance with the requirements [of the first] would necessarily involve election of one or more of the groups set forth [in the second].”
There was, to say the least, some confusion at various points as to how the various claims should be sorted out for purposes of restriction. But even though at some points restriction requirements were imposed that were similar to, or even identical to, earlier restriction requirements, each requirement was nevertheless separately imposed with respect to each separate application. The record thus does not support the inference that any of the various restriction requirements automatically carried forward, in part or in whole, from one application to the next.3 For that reason, we cannot sustain the district *1350court’s summary judgment order, which was based on the court’s conclusion that the 1973 restriction requirement continued in effect with respect to the continuation application that was filed in 1977. Accordingly, we reverse the district court’s judgment and remand for further proceedings.
In light of the complexity of the factual record in this case, we go no further than to address the ground on which the district court ruled. Whether further analysis of the sequence of applications, restriction requirements, and responses by the applicants may reveal other grounds for concluding that the protection of section 121 should be extended to some or all of the claims of the '927 patent is a matter for the district court to address in the first instance.
VACATED and REMANDED.
. The dissent appears to take the position that by issuing the '927 patent the PTO in effect found that the applicant complied with all applicable restriction requirements, and that we should not disturb that determination. In fact, however, the question whether the requirements of section 121 have been satisfied is a question of law that we have addressed de novo after reviewing the relevant materials. See Geneva, 349 F.3d at 1377; In re Berg, 140 F.3d 1428, 1432 (Fed.Cir.1998). The approach suggested by the dissent would be inconsistent with the approach we have employed in similar cases in the past. In Geneva, and Gerber, for example, we held that applicants had failed to satisfy the requirements of section 121 based on our analysis of the prosecution history. Even in cases in which we have held that the requirements of section 121 were satisfied, we did so not as a result of deference to the PTO but as a result of our own analysis of the prosecution history. See Symbol Techs., Inc. v. Opticon, Inc., 935 F.2d 1569 (Fed.Cir.1991); Tex. Instruments Inc. v. U.S. Int’l Trade Comm’n, 988 F.2d 1165 (Fed.Cir.1993).
. Bristol-Myers has not cited any statutory or regulatory basis for concluding that the 1973 restriction requirement was automatically applicable to the '955 continuation. Bristol-Myers cites several cases and a provision (section 201.07) of the 1972 version of the Manual of Patent Examining Procedure ("MPEP”) for the proposition that a continuation application and its parent are "one continuous application, within the meaning of the law.” Godfrey v. Eames, 1 Wall. 317, 68 U.S. 317, 326, *134917 L.Ed. 684 (1863); accord Transco Prods. Inc. v. Performance Contracting, Inc., 38 F.3d 551, 556-57 (Fed.Cir.1994). Those authorities, however, do not support the proposition for which Bristol-Myers cites them. The cases deal only with the issue of priority, and not with PTO procedure for examining a continuation application in light of its parent. Likewise, the cited MPEP section does not address PTO procedure for examining a continuation, but merely sets forth the requirements of a continuation application.
. Pointing to tire examiner’s statement, in an office action on the '706 divisional application, that restriction "has been required” between three categories of inventions, Bristol-Myers argues that the statement indicates the *1350examiner considered that at least some of the restriction requirements from previous applications continued to apply to the later applications. We do not agree with Bristol-Myers' conclusion in that regard. The examiner's isolated use of the present perfect tense in the 1978 office action is not a sufficient basis from which to infer that the examiner understood, or intended to convey, that a restriction requirement imposed five years earlier, in connection with a grandparent application, continued to be in effect for all applications related to the original '989 application because it was never formally withdrawn.