McCarthy and Nugent succeeded in changing the brain of a female rat—after the window had closed—which resulted in the female rat brain taking on the characteristics of a male rat brain. The injections occurred after the first week of birth when the window for brain sexual differentiation was thought to have been closed. Despite this, the preoptic area in these animals was transformed, and took on structural characteristics of a male rat. The female rats also behaved differently, displaying sexual behavior typical of male rats. In another experiment, McCarthy and Nugent genetically deleted the Dnmt gene in female mice. These animals also showed male behavior patterns. Nugent said, “Physically, these animals were females, but in their reproductive behavior, they were males. It was fascinating to see this transformation."