Fc Receptor-Like Protein 1 (FCRL1) is a single-pass type I membrane protein that may function as an activating coreceptor in B cells. FCRL1 is primarily expressed in secondary lymphoid tissues by mature subsets of B cells. It can be detected in the spleen, lymph node, heart, skeletal muscle, kidney, liver and placenta. It is specifically expressed by mature B lineage cells with higher expression at the protein level in naive B cells compared to memory B cells. FCRL1 contains three extracellular C2-like immunoglobulin domains, a transmembrane domain, and a cytoplasmic domain with two immunoreceptor-tyrosine activation motifs and may play a role in the regulation of cancer cell growth.