concurring in part and dissenting in part.
I fully agree with the majority that the judgment of conviction of sexual assault as a part of a pattern of sexual abuse cannot stand, because that conviction may have been based upon one or more incidents occurring prior to the adoption of § 18 — 3—405(2)(c), C.R.S. (1993 Cum.Supp.). It is my view, however, that defendant’s prosecution under this statute suffered from a fundamentally more serious defect because he was not given proper notice of the offense with which he was charged. For this reason, I would reverse the judgment of conviction and remand the cause to the trial court with directions to it to require the People to elect a single incident as the basis for the charge and for a new trial on that charge.
Section 18-3-405(1), C.R.S. (1986 Repl.Vol. 8B) creates the crime of sexual assault on a child. Section 18-3-405(2), C.R.S. (1986 Repl.Vol. 8B and 1993 Cum.Supp.) creates no crime; it is simply a sentence enhancer. See People v. Longoria, 862 P.2d 266 (Colo.1993). Thus, as its specific provisions make clear, the crime described in § 18-3-405(1) is elevated from a class 4 felony to a class 3 felony if the sexual assault charged under that statute is proven to have been committed under certain circumstances, including that circumstance in which the “[t]he actor commits the offense as a part of a pattern of sexual abuse.” Section 18-3-405(2)(c).
In order to establish a “pattern” of sexual abuse, it is not necessary that a specific time or date for each incident comprising the pattern be alleged. It is necessary, however, to establish that the acts constituting the pattern have occurred “within ten years of the offense charged in the information or indictment." Section 18 — 3—405(2)(c). (emphasis supplied)
As I read § 18-3-405, therefore, the People are required to allege and to prove the commission of a particular sexual assault with the same degree of specificity as they are required to allege and to prove the commission of any other offense. To have this singular offense enhanced to the status of a class 3 felony, it is necessary for the People also to allege and to prove that, in addition to *74the predicate offense, at least one additional sexual assault occurred within the pertinent ten-year period.
In my opinion, the People here have never properly charged defendant with a particular sexual assault.
The information that charged defendant with a class 3 felony asserted that defendant had assaulted the victim “on or before October 14, 1990,” as a part of a pattern of sexual abuse. Defendant requested that he be provided with a bill of particulars describing the incident relied upon, as well as the incidents allegedly comprising the pattern of assaults.
In response, the People filed a 4½ page, single-spaced, bill of particulars, describing numerous instances of sexual assaults, starting prior to March 11, 1987, and continuing through October 21, 1991. It was alleged that during the “summer” of 1988, defendant assaulted the victim every Sunday morning. Likewise, it was asserted that there were assaults perpetrated on a regular basis between October 1988 and May 1989. This bill of particulars also described several later incidents.
However, the People did not specify which of these numerous incidents they were relying upon as the assault referred to in the information. Likewise, it did not seek to distinguish between the incident relied upon for proof of the predicate offense and the incident or incidents relied upon as forming a “pattern” of sexual abuse for sentence enhancement purposes.
Indeed, the People never informed defendant which of these numerous incidents was the assault that was referred to in the information. Rather, evidence upon this general pattern of sexual abuse was presented to .the jury, and the jury was instructed in the manner described by the majority. Hence, it was left solely in the hands of the jurors to decide for themselves which of the numerous incidents between early 1987 and late 1990, constituted the predicate offense and which constituted the pattern. Further, because the jury returned only a general verdict, it is impossible even now to determine which incident was found to be the subject of the information’s allegation of sexual assault.
Because the incident selected by the jurors as the predicate offense could have been one that occurred before June 1, 1989, the effective date of § 18-3-405(2)(c), see Colo.Sess. Laws 1989, ch. 163 at 903-904, I agree with the majority that that statute could have been applied retroactively, and therefore, application of this.sentence enhancer to defendant would be improper under these circumstances.
I am also convinced, however, that, under the circumstances disclosed by this record, defendant’s conviction of sexual assault cannot be sustained.
I recognize that due process does not always require that the prosecution specify which of several like incidents is the basis for a charge and that, in appropriate circumstances, the question can be submitted to the jury under an instruction of the nature given here. Thomas v. People, 803 P.2d 144 (Colo.1990).
In Thomas, however, the two victims of the assaults were only 3 and 4 years of age, the incidents took place over a period of only a few weeks, and the nature of the assaults were substantially identical. There, also, the only evidence presented by the People came from the victims who, because of them young ages, had difficulty in distinguishing between the various incidents.
Here, in contrast, the victim was in her teens, the numerous incidents allegedly occurred over a period of some five years, the nature of the assaults differed substantially (ranging from fondling through clothes to full sexual intercourse), and the victim had no trouble in describing the nature of the various incidents, them locations, or their approximate dates, although she was unable to provide an exact date for each incident.
Given these circumstances, I am convinced that due process required that the People provide greater specificity for the charge of sexual assault so as to allow defendant an adequate opportunity to prepare for trial. See Kogan v. People, 756 P.2d 945 (Colo.1988).
That the People could easily have done this here is demonstrated by the fact that they did designate a specific incident as the basis *75for the aggravated incest charge that the trial court dismissed after the return of the verdicts. However, even if the People were unable to designate a specific incident for the sexual assault charge (and this record would, in my opinion, present no basis for such a claim), they should have been required, at the least, to rely upon only a single series of incidents occurring within a limited time frame.
Absent some such designation, however, I would hold that the procedure adopted by the prosecution here violated defendant’s right to due process. Hence, I would reverse defendant’s conviction and remand the cause to the trial court with directions to require the People to elect which incident is the basis for the predicate sexual assault charge and which incident or incidents relate to the “pattern of abuse” sentence enhancement and for a new trial premised thereon.