How do embryos know how to become male or female? Prof. Mike Weiss, chair of the Department of Biochemistry and Molecular Biology at Indiana University, is studying how one protein, known as the sex-determining protein Y, or “SRY,” can program gender. “SRY is like the light switch. The bulb is this downstream developmental pathway that leads to the formation of the organs,” Weiss said. Mammalian embryos are hard-wired to be female, but SRY puts bends in the embryo’s DNA that tell it to shift the development pathway to male. Weiss and his colleagues took a closer look at how this switch worked and they found something suprising. “We wanted to measure the threshold of the SRY directed transcriptional switch for maleness,” Weiss said. “Remarkably, it’s only a factor of two.” This easy-to-flip switch leads to a large range in the amounts of testosterone exposed to the brain during development, which, scientists have speculated, affects the male-ness of the male. “Why would Nature be so