JUN and skin neoplasm: For example, the observed shift in AP-1 composition from fra-1/c-jun to c-fos/c-jun heterodimers in HaCaT keratinocytes after UV-B exposure [10] suggests that after being induced by extracellular and environmental stimuli, c-fos expression and c-fos/c-jun heterodimer formation contribute to the pathological process of HPV positive skin cancer.