It takes a Y chromosome to make a man. But that doesn't mean its counterpart, the X chromosome, plays no part. In fact, it has a big role in making sperm. To study X evolution, researchers first sequenced the human chromosome more accurately than ever before. Then they compared the code to that of a mammal whose evolutionary lineage split from ours about 80 million years ago: the mouse. It turns out that humans and mice share 95 percent of their X genes. This finding was no surprise. In the 1960s, biologist Susumu Ohno suggested the X chromosome would evolve slowly, and thus remain similar in most mammalian species. But some things did change over evolutionary time. Or rather, 144 things. One-hundred-forty-four human X chromosome genes had no counterparts in mice. And these genes seemed to be constantly developing. Many of the genes remained dormant in females, only becoming active in tissues involved in sperm production. The study is in the journal Nature Genetics. Clearly, the X chromosome Xceeds its feminine responsibilities. Which is Xcellent news for male fertility.