For decades scientists thought that the human Y chromosome, the male sex chromosome, was nothing more than a smaller, less stable version of its partner, the X (the sex chromosome present in both females and males). However, new research led by Dr. David Page, member of the Whitehead Institute for Biomedical Research, and associate investigator of the Howard Hughes Medical Institute, reverses this unflattering picture of the Y and reveals it as a crucial player in the evolution of sex chromosomes and also as a safe haven for male fertility genes. These results are not only generating a new respect for the Y chromosome but also could lead to novel diagnostic techniques for thousands of infertile men. The results also have profound implications for understanding the genetic differences between men and women and the genetic underpinnings of chromosomal disorders such as Turner syndrome.