Involved in light regulation of the circadian clock and photomorphogenesis. May play a global role in coordinating growth in response to the light environment. Acts as a light quality sensor directing both negative and positive transcriptional regulation. Inhibits growth in red light but promote growth in blue light. Inhibits clock gene expression in diurnal cycles. Plays no role in the control of flowering time. A number of isoforms are produced. Expressed in leaves stems, flowers, roots, trichomes and hypocotyls. Present in nucleus throughout development. Post transcriptionally regulated. No circadian-regulation at the mRNA level, but fluctuation of the protein levels, with the highest level found shortly after dawn. Shortened circadian period. The clock is hypersensitive to red but shows normal responses to blue light. By contrast, inhibition of hypocotyl elongation is hyposensitive to red light but hypersensitive to blue light. Belongs to the FAM50 family.