PCNA and neoplasm: NKp44 ligands include the following [42]: (1) tumor cell ligands, such as proliferating cell nuclear antigen (PCNA), platelet-derived growth factor DD (PDGF-DD), nidogen-1, and NKp44L (NKp44L, an isomer of mixed-lineage leukemia-5 protein (MLL5), is expressed in tumor cells and transformed cells that can improve cell sensitivity to the cytotoxicity of NK cells [56]); (2) viral ligands, such as HA and HN; and (3) bacterial ligands, such as Mycobacterium tuberculosis cell wall components.