KLRK1 and neoplasm: We also noted that expression of some classical MHC class I molecules (H2-Db, H2-Kb, and H2-M3) was increased in the tumor-forming Sp17+ cell population, but expression of non-classical MHC I molecules (e.g., Qa-2) remained unchanged, as did the expression of rae-1, a ligand for the NK cells stimulatory receptor NKG2D.