Sunlight, vitamin A and the skin.
D000818:Animals; D006801:Humans; D011817:Rabbits; D012176:Retinoids / Q000528:radiation effects; D012177:Retinol-Binding Proteins / Q000032:analysis; D012867:Skin / Q000032:analysis / Q000528:radiation effects*; D013472:Sunlight; D014466:Ultraviolet Rays; D014801:Vitamin A / Q000097:blood / Q000528:radiation effects*
D012176:Retinoids; D012177:Retinol-Binding Proteins; D014801:Vitamin A