Many genes on the Y chromosome are involved in male sex determination and development. In particular, genes in areas of the Y chromosome called azoospermia factor (AZF) regions provide instructions for making proteins that are involved in sperm cell production and development, although the specific functions of these proteins are not well understood. Y chromosome infertility is caused by deletions of genes in the AZF regions. These deletions remove several genes, or in rare cases, a single gene. Loss of this genetic material likely prevents the production of one or more proteins needed for normal sperm cell development. As a result, either a few sperm develop or sperm do not develop at all, leading to Y chromosome infertility.