The multigene family of Fc receptor homologs (FcRHs,CD307) have variable numbers of extracellular immunoglobulin domains of five different subtypes. FcRH immunoregulatory potential is implicated by the presence of consensus tyrosine-based activation or inhibition motifs in their cytoplasmic tails. The protein products of the human and mouse FcRH5 genes, which are also designated Fc receptor-like protein 5, differ in that human FcRH5 comprises 977 amino acids, whereas mouse FcRH5 contains 582 amino acids.