Loss-of-function mutations in the filaggrin gene (FLG) are observed in approximately 10% of Northern Europeans. Heterozygous and homozygous mutation-carriers have, respectively, partial or complete reduction of epidermal filaggrin and its degradation products. Among others, filaggrin is degraded to trans-urocanic acid (UCA)(1) ; a chromophore that provides protection against ultraviolet (UV) radiation and modulates cutaneous immune functions(1) . This article is protected by copyright. All rights reserved.