Normal p53 protein protects the genome after DNA damage by delaying replication and allowing DNA repair, mdm‐2 is a recently discovered protein that controls p53 activity by binding to and inactivating p53 protein. A negative feedback loop has been suggested in which p53 induces mdm‐2 expression. We have shown that doses of ultraviolet B radiation that cause mild sunburn clinically, produce thymine dimers in keratinocytes detectable by immunocytochemistry. This causes an elevation of p53 protein without a concomitant p53‐mediated induction of