PCNA and neoplasm: Interestingly, PCNA was recently described as an unexpected inhibitory ligand for the NCR NKp44; an over-expression of PCNA was found to inhibit NK cell cytotoxicity in an NKp44-dependent manner, revealing a putative novel mechanism for pathologic cells to evade NK attack (53, 54), closed to the numerous mechanisms developed by tumor and infected cells to counteract their sensitivity to NK cells (55).