ATF3 and melanoma: We then investigated whether imiquimod‐induced ER stress of the two melanoma cell lines is associated with activation of PERK, inositol‐requiring‐1α (IRE1α), activating transcription factor 4 (ATF‐4), the unfolded protein response (UPR) regulator (GRP78), C/EBP‐homologous protein (CHOP), calpain and caspase‐4 or ATF‐3.