higher plants ( 13), should lead to the cloning of more clock components. Will some of these contain PAS domains, making the use of this motif universal? Second, are proteins such as WC-1 and WC-2 (or even PER) also photoreceptor molecules themselves? Macino and his colleagues have speculated this could be so, on the basis of the homology of the PAS region to part of the chromophore-binding region of PYP (7). Modeling of PAS domains based on the PYP struc-ture may indicate whether the domain is at 3. S. A. Kay and A.