Figure 1. Mouse cells deficient in clock genes are not sensitive to UV irradiation. Cells were plated at a low density, allowed to attach and then UV irradiated; after colony formation, cells were stained with Giemsa, and percent survival was determined as described in the text. Each data point represents the average of at least three independent experiments and bars signify the standard deviation. (A) Positive arm of the transcription-translational feedback loop (TTFL). Mouse embryonic fibroblasts were derived from mice with the following genotypes: wild-type (squares), Clock^−/− (circles) and Bma1l^−/− (triangles). (B) Negative arm of the TTFL. Mouse skin fibroblasts were derived from mice with the following genotypes: wild-type (squares), Cry1^−/−Cry2^−/− (circles) and Per1^−/−Per2^−/− (triangles).