PCNA and neoplasm: Activating receptors include (1) Ly49, a C-type lectin receptor, which identifies the “self” marker of major histocompatibility complex (MHC) class I [6]; (2) natural cytotoxicity receptors (NCRs) (NKp30, NKp44, and NKp46); (3) transmembrane proteins that bind viral and bacterial ligands as well as tumor cell ligands such as proliferating cell nuclear antigen (PCNA) [7]; and (4) CD16 (FcγIIIA), which binds immunoglobulin (Ig) G for antibody-dependent cell-mediated cytotoxicity (ADCC) [8].