Cryptochromes--bringing the blues to circadian rhythms.

Cryptochromes are blue/UV-A-absorbing photoreceptor proteins discovered originally in plants and so named because their nature proved elusive in over a century of research. Now we know that the photoreceptor essential for proper seedling establishment in blue light has homologues in the animal kingdom - in insects, in mice and in humans. In recent months, evidence has emerged pointing to a common role for cryptochromes in all of these organisms in entraining the circadian clock, a biochemical timing mechanism running within cells, synchronizing metabolism to the daily light-dark cycle and having consequences on a much larger scale in the regulation of behaviour such as the sleep-wake cycle.