Table 1.

Circadian clock genes and their functions in Drosophila and mice

Organism	Gene	Protein function	Clock function
Drosophila	per	Binds to dClk-Cyc as a heterodimer with Tim	Inhibits dClk-Cyc-dependent transcription
	tim	Stabilizes phosphorylated Per	Contributes to delayed Per accumulation
		Promotes Per nuclear localization	Acts with Per to inhibit dClk-Cyc-dependent transcription
		Binds to dClk-Cyc as a heterodimer with Per	Mediates phase-resetting in response to light
		Degraded in response to light
	dClk	Forms a heterodimeric complex with Cyc	Activates per and tim transcription and inhibits dClk transcription
	Cyc (Bmal1)	Forms a heterodimeric complex with dClk	Activates per and tim transcription and inhibits dClk transcription
	dbt (CK1ε)	Phosphorylates Per	Destabilizes Per
	cry	Photoreceptor element that binds to Tim	Initiates light signaling required for phase resetting
Mouse	mPer1	Interacts with Clock-Bmal1?	Moderate inhibition of Clock-Bmal1-dependent transcription
	mPer2	Interacts with Clock-Bmal1?	Activation of Bmal1 transcription
			Moderate inhibition of Clock-Bmal1-dependent transcription
	mPer3	Interacts with Clock-Bmal1?	Moderate inhibition of Clock-Bmal1-dependent transcription
	mTim	Interacts with Clock-Bmal1?	Moderate inhibition of Clock-Bmal1-dependent transcription
	Clock	Forms a heterodimeric complex with Bmal1	Activates mPer1-3 and mCry1
	Bmal1	Forms a heterodimeric complex with Clock	Activates mPer1-3 and mCry1
	CK1ε (tau)	Phosphorylates mPer1 and mPer2	Destabilizes mPers
	mCry1	Dimerizes with mPer1-3	Strongly inhibits Clock-Bmal1-dependent transcription
		Binds to Clock-Bmal1
		Promotes mPer1-3 nuclear localization
	mCry2	Dimerizes with mPer1-3	Strongly inhibits Clock-Bmal1-dependent transcription
		Binds to Clock-Bmal1
		Promotes mPer1-3 nuclear localization

Gene symbols are as described in the text. Protein functions are derived from in vivo and/or in vitro experiments. Clock function is derived from mutant phenotypes and/or in vivo experiments. The '?' for mPer1-3 and mTim protein function indicates that the indicated activity is presumed on the basis of the clock function of these proteins. Cyc is the Drosophila homolog of mouse Bmal1 and dbt is the homolog of mouse CK1ε and hamster tau.