I have spent my entire research career investigating X inactivation -the compensatory mechanism for equalizing the sex difference in the number of X-chromosomes in mammals. This adjustment, which turns down the expression of genes on the X chromosome in human females, is absolutely essential for their survival, and has a major role in female sex determination. Although much has been written about the Y chromosome and its role in inducing maleness, very little has been written about the blueprints for female development. And discussions of the basis for sex differences in disease are usually limited to hormones or body size. Up to now, there has been no book-length discussion of X inactivation. My studies and those of others shed significant light on the molecular mechanisms underlying this developmental process. They also reveal the genetic and medical consequences of turning off one X chromosome in female cells. It is time to put together, within a single volume, the large amount of scientific material now available in research publications about this early embryonic event, and how it influences the health of women. I propose to prepare a book-length manuscript about sex dosage compensation in human females with in-depth discussions ranging from the molecular basis to the effect on women's health. This will be the first of its kind. Such information will be of value to biologists, medical scientists, medical students, physicians and other health professionals interested in the special aspects of health in females.