– CD300A is a single-pass type I membrane protein which belongs to the CD300 family. It contains 1 Ig-like V-type (immunoglobulin-like) domain. The CD300 family of myeloid immunoglobulin receptors includes activating (CD300b, CD300e) and inhibitory members (CD300a, CD300f), as well as molecules presenting a negative charge within their transmembrane domain (CD300c, CD300d). It is expressed not only by natural killer (NK) cells but also by T-cell subsets, B-cells, dendritic cells, mast cells, granulocytes and monocytes. CD300A is an inhibitory receptor which may contribute to the down-regulation of cytolytic activity in natural killer (NK) cells, and to the down-regulation of mast cell degranulation. CD300c is a functional immune receptor able to deliver activating signals upon ligation in RBL-2H3 mast cells.