Does a requirement that a defendant must register as a sex offender constitute punishment for purposes of ex post facto analysis?

As noted above, we held in People v. McVickers, supra, 4 Cal.4th 81, 88, 13 Cal.Rptr.2d 850, 840 P.2d 955, that requiring a defendant to provide a blood sample for AIDS testing imposes a burden, but is not punitive in nature. We held that the statutes there under review "have a legitimate purpose other than punishment ... [and] the method chosen a mandatory AIDS test with results given only to the defendant and to law enforcement officials is not excessive in relation to the statute's asserted purpose." (Id. at p. 89, 13 Cal.Rptr.2d 850, 840 P.2d 955.) Similarly, in the present case, requiring a defendant to register as a sex offender certainly imposes a burden, but whether the registration requirement constitutes punishment for purposes of ex post facto analysis depends upon whether the purpose or effect of this requirement is punitive in nature.