The phototropins phot1 and phot2 are plant blue-light receptors that mediate phototropism, chloroplast movements, stomatal opening, leaf expansion, the rapid inhibition of hypocotyl growth in etiolated seedlings, and possibly solar tracking by leaves in those species in which it occurs. The phototropins are plasma membrane-associated hydrophilic proteins with two chromophore domains (designated LOV1 and LOV2 for their resemblance to domains in other signaling proteins that detect light, oxygen, or voltage) in their N-terminal half and a classic serine/threonine kinase domain in their C-terminal half. Both chromophore domains bind flavin mononucleotide (FMN) and both undergo light-activated formation of a covalent bond between a nearby cysteine and the C(4a) carbon of the FMN to form the signaling state. LOV2-cysteinyl adduct formation leads to the release downstream of a tightly bound amphipathic 冄-helix, a step required for activation of the kinase function. This cysteinyl adduct then slowly decays over a matter of seconds or minutes to return the photoreceptor chromophore modules to their ground state. Functional LOV2 is required for light-activated phosphorylation and for various blue-light responses mediated by the phototropins. The function of LOV1 is still unknown, although it may serve to modulate the signal generated by LOV2. The LOV domain is an ancient chromophore module found in a wide range of otherwise unrelated proteins in fungi and prokaryotes, the latter including cyanobacteria, eubacteria, and archaea. Further general reviews on the phototropins are those by Celaya and Liscum (2005) and Christie and Briggs (2005).Author for correspondence. Tel: +1 650 325 1521 (extension 207); Fax: +1 650 325 06857; E-mail: briggs@stanford.edu