PCNA and melanoma: The increase in PCNA+/TYRP1+ cells within the RXRαep−/−|Tyr-NRASQ61K|Cdk4R24C/R24C skin as compared to the RxrαL2/L2 control mice indicated the presence of a large volume of proliferating melanocytes in the mutant melanomas in absence or in presence of acute UVB.