Killer cell lectin-like receptor G1 (KLRG1) is the mouse homolog of the rat mast cell function-associated antigen (MAFA or 2F1-Ag). KLRG1 is a type II membrane glycoprotein that was first identified on the surface of rat mast cell line RBL-2H3. It is composed of a homodimer of glycosylated 30-38 kD subunits. Mouse and human homologs of KLRG1 are expressed by subsets of NK cells and lymph okine-activated killer (LAK) cells but not mast cells. KLRG1 is also expressed on subsets of CD8+ and CD4+ cells, including CD4+ and CD8+ effector/memory cells, potent regulatory CD4+ T cells. KLRG1 may be involved in regulating NK cell homeostasis. KLRG1 was found to recognize cadherins and thus inhibit immune responses by regulating the effector function and the developmental processes of NK and T cells.