ULBP-1, -2, and -3 were originally identified as ligands for the human cytomegalovirus glycoprotein UL16 and designated UL16-binding proteins (ULBP). These proteins are distantly related to major histocompatibility class I (MHC I) molecules, possessing the alpha 1 and alpha 2 Ig-like domains, but lacking the alpha 3 domain. Unlike MHC Class I, they have no capacity to bind peptide or interact with beta2-microglobulin. ULBP-1, -2, and -3 are known to bind to human NKG2D, an activating receptor expressed on NK cells, NKT cells, gamma delta T cells, and CD8+ alpha beta T cells.