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L. J., was abducted while she was walking home from work. The perpetrator forced her into his car and raped her, then robbed her of her money and other personal items and pushed her out into the street. L. J. ran home and reported the attack to her mother, who called the police. An ambulance took L. J. to the hospital, where doctors treated her wounds and took a rape kit. The police labeled it with an inventory number and sent it under seal to the Illinois State Police (ISP) lab. A forensic scientist received the sealed kit and conducted a chemical test that confirmed the presence of semen on the vaginal swabs, and he then resealed the kit and placed it in a secure evidence freezer. The ISP lab sent L. J.'s vaginal swabs to Cellmark for testing, and Cellmark sent back a report containing a male DNA profile produced from semen taken from those swabs. D was not under suspicion for L. J.'s rape. Sandra Lambatos, a forensic specialist at the ISP lab, conducted a computer search to see if the Cellmark profile matched any of the entries in the state DNA database. The computer showed a match to a profile produced by the lab from a sample of D's blood that had been taken after he was arrested on unrelated charges on August 3, 2000. The police conducted a lineup, and L. J. identified D as her assailant. D chose to be tried before a state judge. Lambatos testified as an expert witness in forensic biology and forensic DNA analysis. P asked Lambatos, “Did you compare the semen that had been identified by Brian Hapack from the vaginal swabs of [L. J.] to the male DNA profile that had been identified by Karen [Abbinanti] from the blood of [petitioner]?” Lambatos answered “Yes.” D objected, and it was overruled. Lambatos then testified that, based on her own comparison of the two DNA profiles, she “concluded that D cannot be excluded as a possible source of the semen identified in the vaginal swabs,” and that the probability of the profile's appearing in the general population was “1 in 8.7 quadrillion black, 1 in 390 quadrillion white, or 1 in 109 quadrillion Hispanic unrelated individuals.” Id., at 57. Asked whether she would “call this a match to [petitioner],” Lambatos answered yes, over D's objection. On cross, Lambatos confirmed that she did not conduct or observe any of the testing on the vaginal swabs and that her testimony relied on the DNA profile produced by Cellmark. The Cellmark report was never entered into evidence. D was found guilty and appealed. Eventually, the Supreme Court granted certiorari.

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