Mouse Fc receptor-like protein 1 (FCRL1) is a single-pass type I membrane protein. It is expressed in B-cells at the various stages of differentiation. Mouse FCRL1 consists of a 203 amino acid (aa) extracellular domain (ECD) with two Ig-like domains, a 21 aa transmembrane segment, and a 103 aa cytoplasmic domain with two immunotyrosine activation motifs (ITAMs). FCRL1 is likely not a receptor for immunoglobulin. It may function as an activating coreceptor in B-cells and B-cells differentiation.