Qa-1, a nonclassical class I histocompatibility molecule encoded by the T23 gene in the T region expressed in mice, predominantly assembles with a single nonameric peptide, Qdm, derived from the signal sequence of certain class Ia molecules. The Qa-1/Qdm complex serves as a ligand for CD94/NKG2 natural killer (NK) receptor(s) express on a major fraction of NK cells and CD8+ immuno-suppressive regulatory T cells. Qa-1, expressing on immuno-regulatory B cells and other antigen presenting cells, serves for antigen-presentation to immuno-regulatory CD8+ T.