The phosphorylation defective RARα mutant inhibits AP-1 activity in the absence of RA. Triplicate cultures of SCC25 cells were transiently transfected with a heterologous promoter construct containing an AP-1 response element in the luciferase reporter vector pGL3 as described in Methods. Wild type (alpha) or S77A mutant RARα expression vectors were cotransfected with the reporter constructs. Blank expression vector was used to control for the amount of DNA transfected. Transfected cultures treated with vehicle or 1 μM RA for 24 hours were used as controls for AP-1 inhibition. Relative light units from vehicle treated cells transfected with reporter construct and blank vector were assigned an arbitrary AP-1 activity value of 100. These experiments were performed three times with similar results. Error bars indicate SEM.