Serum-induced mPer1 gene expression is elevated in Id2−/− mouse embryonic fibroblasts. Serum stimulation of MEFs was used as a model system to mimic photic stimulation of the SCN. A, relative expression of c-fos in Id2−/− compared with WT (Id2+/+) MEFs shows no significant difference between genotypes. B, mPer1 expression is 2-fold higher in the peak level of gene expression in Id2−/− cells at 60 min post-initiation of serum treatment. C, relative expression of mPer2 shows no significant difference between cells. Values are mean ± S.E. fold change of expression relative to the lowest expression value from five independent experiments. c-fos, mPer1, and mPer2 expression was assessed by qRT-PCR using SYBR green and normalized to GAPDH assessed using Taqman reagents. Two-tailed Student's t test was performed at individual time points to determine statistical significance between WT and Id2−/− samples. **, p value < 0.01.