The circadian rhythms in mammals are regulated by a pacemaker located in the suprachiasmatic nucleus of the hypothalamus^,. Four clock-gene families have been found to be involved in a transcription–translation feedback loop that generates the circadian rhythm at the intracellular level. The proteins Clock and Bmal1 form a heterodimer which activates the transcription of the Per gene from the E-box elements in its promoter region^,. Protein products of Per act together with Cry proteins to inhibit Per transcription^,, thus closing the autoregulatory feedback loop. We found that Dec1 and Dec2, basic helix–loop–helix transcription factors, repressed Clock/Bmal1-induced transactivation of the mouse Per1 promoter through direct protein–protein interactions with Bmal1 and/or competition for E-box elements. Dec1 and Dec2 are expressed in the suprachiasmic nucleus in a circadian fashion, with a peak in the subjective day. A brief light pulse induced Dec1 but not Dec2 expression in the suprachiasmic nucleus in a phase-dependent manner. Dec1 and Dec2 are regulators of the mammalian molecular clock, and form a fifth clock-gene family.