Signal-regulatory protein beta-1 (SIRP beta 1) also known as SIRP?1, belongs to signal-regulatory-protein (SIRP) family, and immunoglobulin superfamily. SIRP?1 is a transmembrane protein that has three Ig-like domains in its extracellular region and a short cytoplasmic tail. Human SIRP?1 is expressed on monocytes and granulocytes but not on lymphocytes. SIRP beta 1 contains short cytoplasmic domains that lack cytoplasmic sequence motifs capable of recruiting SHP-2 and SHP-1. In addition, they contain a single basic lysine residue within the hydrophobic transmembrane domain.