Id4-null mice exhibit abnormally elevated daytime gene expression of Id1 within the suprachiasmatic nucleus. Quantative gene expression assessment of Id1, Id2, and Id3 in wild-type and Id4−/− mice SCN. Mice were extrained to a 12:12 LD cycle and transferred to DD before time-specific assement. qRT-PCR analysis of time-specific SCN tissue punches, collected at (a) Circadian time (CT) 6 (subjective daytime, wild type (Id4+/+) n = 8, Id4−/− n = 7) and (b) CT20 (subjective night, wild type n = 10, Id4−/− n = 12). Values are mean ± SEM relative gene expression values (relative to acidic ribosomal protein, ARP expression). * p < 0.05.