Fc receptor-like protein 1, also known as FcR-like protein 1, Fc receptor homolog 1, IFGP family protein 1, Immune receptor translocation-associated protein 5 and FCRL1, is a single-pass type I membrane protein which contains threeIg-like C2-type (immunoglobulin-like) domains. It is a cell-surface membrane protein belonging to FCRL family and is preferentially expressed on B cells. FCRL1 is primarily expressed in secondary lymphoid tissues by mature subsets of B cells. It is detected in spleen, lymph node, heart, skeletal muscle, kidney, liver and placenta. FCRL1 is specifically expressed by mature B lineage cells with higher expression in naive versus memory B cells (at protein level). Human Fc receptor-like molecules (FCRL1, FCRL2, FCRL3, FCRL4, FCRL5) have tyrosine-based immunoregulatory potential and are expressed by B-lineage subpopulations. FCRL1 may function as an activating coreceptor in B cells. It may also function in B cells activation and differentiation.