MDM4 and actinic keratosis: Through drug repurposing approach, we have previously shown that small molecule protoporphyrin IX (PpIX), a drug approved to treat actinic keratosis, binds to the p53 N-terminus and disrupts p53/MDM2 and p53/MDMX complexes (Zawacka-Pankau et al., 2007; Sznarkowska et al., 2011; Jiang et al., 2019).