PCNA and neoplasm: Interestingly, other cellular and tumor ligands have been reported to bind and regulate NKp44 signaling, such as Nidogen-1 (101), Syndecan-4 (102), a subset of HLA-DP molecules (103), a splice variant of the mixed lineage leukemia 5 (MLL5) gene (104), and proliferating cell nuclear antigen (PCNA) (105).