NK1.1, a member of the NKRPI family of cell surface receptors, is a type II integral membrane protein with a C type lectin domain. It is expressed as a disulfide linked homodimer on all NK cells, as well as subsets of thymocytes and peripheral T lymphocytes in selected strains of mice (e.g., C57/BL6, FVB/N, NZB; but not A, AKR, BALB/c, CBA/J, C3H, C57BR, C58, DBA/1, DBA/2, SJL, 129). NK1.1 mediates cellular activation and differentiation, and is thought to have a particular role in generating Th2 cells.