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cryptochrome A plant pigment that absorbs mainly blue and ultraviolet (UV-A) light and is thought to be involved in various plant responses to light, including phototropism and hypocotyl elongation. The chemical nature of cryptochromes has proved elusive (hence the name, which means ‘hidden pigment’), but recently a blue receptor in Arabidopsis has been characterized as a protein containing two light-absorbing centres; one is the riboflavin derivative FAD, and the other is a pterin. It is thought that other cryptochromes may also be flavoproteins.