(A) In response to blue light (BL), the CRY monomers undergo photochemical reduction in the flavin cofactor (FAD), followed by dimerisation and formation of an active dimer. (B) Domain overview of cryptochromes and BICs. (C) The CRY PH domain can interact with the BIC CID domain, forming an inactive CRY-BIC heterodimer. This prevents the homodimerisation and subsequent activation of CRYs. (D) In wild-type plants (first panel), exposure to blue light triggers the de-etiolation process and stops the hypocotyl from elongating. The cry1/cry2 double mutant (second panel) leads to elongated hypocotyls, while the overexpression of CRY1 (third panel) results in shortened hypocotyls. The bic1/bic2 double mutant (fourth panel), which is unable to repress cryptochrome function, has a phenotype similar to the CRY1 overexpression mutant, while the overexpression of BIC1 (fifth panel) has a similar phenotype to the cry1/cry2 double mutant.