Dr. Margaret McCarthy collaborated with Bridget Nugent, PhD, now a researcher at the University of Pennsylvania to investigate how gender is determined in the brain. They discovered that inhibiting DNA methyltransferases or Dnmts, which modify DNA to repress gene expression, has powerful effects. Typically, there is a designated time frame during gestation in which the brain takes on male or female characteristics. The understanding has been that once this window closed, it could not be reopened. McCarthy and Nugent injected Dnmt inhibitors into a specific region known as the preoptic area, or POA. In every species that's been studied, including humans, the POA plays a key role in governing male sexual behavior. What happened? Part 4 tomorrow.