Shedding light on photoperiodism.

I n mammals, photoreception is restricted to the eye. By contrast, nonmammalian vertebrates possess photoreceptors in a wide array of extraocular tissues, including the pineal complex and brain (1). The identity of the deep brain photoreceptors has remained elusive. They continue to be the least studied of all nonvisual photoreceptor classes, although their existence in the vertebrates has been known fornearly a century. In this issue of PNAS, the Yoshimura laboratory presents a study that represents a significant stride toward identifying and characterizing encephalic photoreceptors (2).