TP53 and neoplasm: The p53 co-activator Zac1 functions as a tumour suppressor, and reduced or absent Zac1 expression has been frequently observed in a number of different types of tumours.15–18 This is supported by results where cultured retinal explants from Zac1 null allele mice showed an increase in the number of cells in the inner and outer nuclear layer of the retina.22 The inability of Zac1 to arrest the cell cycle of HPCs further strengthens the notion that the HCs are less sensitive to signals that regulate cell cycle progression.