Fc receptor homologs, also known as IRTA or IFGP (IgSF, FcR, gp42) proteins, are transmembrane proteins with sequence homology to classical Fc receptors. FcRH have from three to nine Ig-like domains in their extracellular region and variable combinations of cytoplasmic immunotyrosine activation or inhibitory motifs (ITAM or ITIM). FcRH function as activating or inhibitory receptors on discrete B cell populations and are involved in lymphocyte development and transformation. No ligands for these proteins have been identified.