Abstract: The invention provides isolated nucleic acids molecules, designated Siva nucleic acid molecules, which encode proteins involved in immune cell apoptosis. The invention also provides antisense nucleic acid molecules, recombinant expression vectors containing Siva nucleic acid molecules, host cells into which the expression vectors have been introduced, and nonhuman transgenic animals in which a Siva gene has been introduced or disrupted. The invention still further provides isolated Siva proteins, fusion proteins, antigenic peptides and anti-Siva antibodies. Diagnostic, screening, and therapeutic methods utilizing compositions of the invention are also provided.