Hey, do you remember the name of the episode where the killer had a genetic disease that changed her dna and the team couldn't arrest her at first? (watch description plz)

She was a woman, in season 3 (or maybe 4)and she entered the car where don and jessica were in and they got her hair but at the begining it didn't match with the killer's but in the end they arrested her. I gave all the info that i remember i need the answer quickly!!