Photoaging and skin cancer can result from the exposure of skin to ultravio
let A (UV-A, 320-400 nm) radiation. The detailed chemical mechanisms by whi
ch these processes occur are not known, but they must begin with the absorp
tion of a UV-A photon by one or more photoreceptor(s) within the skin. The
situation is complicated by the lack of understanding of the photoreactions
of many of the UV-A-absorbing molecules in skin. In this Account, we descr
ibe recent research efforts directed at elucidating the UVA-induced photore
activity of two light-absorbing epidermal photoreceptors: trans-urocanic ac
id and eumelanin.