Genomic DNA samples collected from mouse (C57BL/6J), human (D. P. K.), chicken, golden hamster (Mesocricetus auratus), lizard (Anolis carolinensis), frog (Xenopus laevis), and zebrafish (Danio rerio) were digested with PstI and analyzed by Southern blot using a mouse Clock PAS-B PCR probe (see Experimental Procedures). Similar results were obtained with a C-terminal probe (data not shown).