Epidemiologic studies have revealed that many cancer types display differences in incidence or outcomes between the sexes. In most cases, these differences are only partially explained by non-genetic factors such as hormonal differences, carcinogen exposure, lifestyle, and access to health care. Our understanding of how genetic factors contribute to differences in cancer incidence between the sexes remains incomplete. A fundamental genetic difference between the sexes is in chromosome composition. Relative to male somatic cells, female somatic cells have an extra X chromosome. Most genes on the second copy of chromosome X in females are inactivated via a process known as X-chromosome inactivation, which approximately equalizes the dosage of X-linked genes between males and females. Dr. Viswanathan's project tests the hypothesis that genetic alterations to the X chromosome in cancer may perturb this carefully regulated process and thereby contribute to differences in cancer incidence or pathogenic mechanisms between males and females.