Signal regulatory proteins of the α subtype (SIRPα) are ubiquitous molecules of the
immunoglobulin superfamily that negatively regulate protein tyrosine kinase receptor-
dependent cell proliferation. Their intracytoplasmic domain contains four motifs that
resemble immunoreceptor tyrosine-based inhibition motifs (ITIMs) and that, when tyrosyl-
phosphorylated, recruit cytoplasmic SH2 domain-bearing protein tyrosine phosphatases
(SHPs). ITIMs are borne by molecules that negatively regulate cell activation induced by …