Urocanic acid in the skin: a mixed blessing?

Located in the stratum corneum, urocanic acid is a major epidermal chromophore for UVR. This simple molecule has attracted a great deal of research interest over the past half century, initially as a putative "natural sunscreen" and later as a mediator of photoimmunosuppression with a consequent role in photocarcinogenesis. For the first time, Barresi and colleagues provide robust evidence for the photoprotective role of endogenous urocanic acid and reopen the debate on the relative "beneficial" and "detrimental" properties of this molecule.