Terrein is a polar, substituted dihydrocyclopentenone first reported by Raistrick and colleagues in the 1930s and subsequently found in a number of Aspergillus and Penicillium species. For many years the biological profile and mode of action of terrein was unstudied; however, recent research showed that terrein reduces melanin synthesis by reducing tyrosinase production via ERK activation, followed by MITF down-regulation. Terrein also reduces human keratinocyte proliferation by inhibiting ERK and by decreasing the expression of cyclin B1 and Cdc2 complex.