As director of Boston University’s biomedical forensic sciences program, she teaches her students to be unequivocally ethical, and exacting when it comes to protocol. “Forensic means the application of science to the law. Your science has to be good, and your ability to testify has to be good. If only one of these works, that’s a breakdown of the system,” says Cotton, who has been working in the field for more than three decades — she started before molecular biology was used as evidence in crime scenes. She began her career at the National Institutes of Health, researching genes on the Y chromosome. Then she happened upon a research paper on how DNA could be used for sexual assault evidence. “I thought, ‘That’s a really great thing to be able to do,’ ” says Cotton, who was one of the first to apply DNA testing at a private crime lab — a practice soon employed by the FBI.