The periodicity of the hair growth cycle is reminiscent of other
cyclic processes, such as the circadian rhythm where distinct clock
mechanisms exist. The regulation of circadian rhythm consists of
positive and negative regulatory feedback loops, having a period of
approximately 24 hours [10–12]. The positive feedback loop is
regulated by two DNA-binding basic helix-loop-helix (bHLH) PAS
domain transcription factors CLOCK and BMAL1 [13], which
form a heterodimer and transcriptionally activate genes containing
E-boxes in their regulatory regions. Among their targets are PER
(PER 1, 2 and 3) and CRY (CRY1 and 2) that form heterodimers
and repress their own transcription via direct interactions with
CLOCK/BMAL1 [14]. Additional targets include RORs (Rora,
Rorb, and Rorc) and REV-ERBs (Nr1d1 and Nr1d2), which are
members of a subfamily of orphan nuclear receptors that bind to
ROR response elements (RREs) to transcriptionally activate and
repress Bmal1, respectively [15,16]. Other CLOCK-controlled
targets, such as the transcription factors DBP and TEF, are not