Born to Kill

S5 E24: When a serial killer suspected of killing young women and branding them with a "Y" is caught and identified as having the "criminal gene," the team feels that they have closed their case until the killings begin again. Evidence originally leads the CSIs to a suspect with the rare extra "Y" chromosome, a condition known in the 70's as the "criminal gene" and believed to make a person more aggressive. But while investigating the Nature vs. Nurture element of the case, the killer escapes and Horatio must race to find him again before it's too late.