Mice used in each experiment are being reared under controlled light-dark conditions. When the light-dark cycle is 12/12 hours, the starting time of the light period is “ZT0” and the starting time of the dark period is “ZT12”. “ZT” is the abbreviation for “Zeitgeber Time”. “Zeitgeber” in German is “Time giver” in English and refers to the “agent of entrainment”.
BMAL1/CLOCK heterodimer binds to the regulatory regions (E-box) of the Period (Per1 and Per2) and Cryptochrome (Cry1 and Cry2) genes and positively regulates their transcription. Transcribed and translated PER and CRY proteins also form a heterodimer, translocate to the nucleus, and negatively regulate the function of the BMAL1/CLOCK complex forming a negative feedback loop.
It is known that PER and CRY are respective members of their own family of factors. Even if one gene loses its function, other members in the same gene family can compensate the resulting loss of function.