Urocanic acid, produced in the upper layers of mammalian skin, is a major absorber of ultraviolet radiation (UVR). In Vitro: Urocanic acid (UCA) is formed in the upper layers of the epidermis where filaggrin, a histidine-rich filamentous protein produced after caspase-14 cleavage of profilaggrin, is broken down by proteinases into component amino acids^[1].