KLRK1 and neoplasm: In contrast, in pathological conditions, virally infected or tumor-transformed cells usually lack/down-regulate the expression of HLA class I molecules (“missing-self hypothesis”) [6], to avoid attack mediated by CD8+ T cells, and acquire/increase the expression of the ligands for the non-HLA specific activating NK receptors (such as NCRs and NKG2D), thus allowing the NK cell-mediated killing of the diseased cells.