The Y chromosome is an isolated piece of genetic material whose only known function is regulating sexual determination and gametogenesis. Very little is understood concerning the mechanism whereby the Y carries out these functions or those related specifically to its existence as a chromosome, such as mitosis and meiosis. The situation is exacerbated by the fact that there is a severe lack of genetic markers on the Y chromosome. We propose to explore the structure and function of the mouse Y chromosome at the level of the DNA sequences that compose it. We have worked out a screening procedure that allows the identification of clones of DNA that contain male-specific sequences, and have applied it to isolate fragments that hybridize to male DNA and fails to do so with DNA from female mice. The principal goals of this proposal are to: (a) isolate a minilibrary of such male-specific DNA and confirm its location on the Y chromosome by genetic and in situ chromosomal studies; (b) characterize the sequence structure of these clones with regard to the sequences shared with other chromosomes, the male specific sequences unique to the Y, the copy number of each and relative positions of the two types of sequence with each clone; (c) compare characteristics of these sequences among the Y chromosomes from various mutants and strains known to be altered in specific aspects of Y chromosome biology; and, (d) determine whether any of these sequences are expressed. Once established and characterized as described above, this set of clones will have provided some very basic information about the organization and function of the Y chromosome and will provide a valuable tool for future studies in the areas of chromosome biology and sex determination.