The relatively limited extent of the SCCs observed in the SKH-hr1 mice compared to the SKH-hr2+ApoE, SKH-hr2 and Nude mice, alongside the low rate of precancerous conditions such as actinic keratosis in the SKH-hr1 mice, further illustrates the variable susceptibility and response of different genetic backgrounds to carcinogenic stimuli. This evidence concerns the gene APOE and actinic keratosis.