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The molecular components of mammalian circadian clocks are elusive. We have isolated
a human gene termed RIGUI that encodes a bHLH/PAS protein 44% homologous to Drosophila
period. The highly conserved mouse homolog (m-rigui) is expressed in a circadian pattern
in the suprachiasmatic nucleus (SCN), the master regulator of circadian clocks in
mammals. Circadian expression in the SCN continues in constant darkness, and a shift
in the light/dark cycle evokes a proportional shift of m-rigui expression in the SCN.
m-rigui transcripts also appear in a periodic pattern in Purkinje neurons, pars tuberalis,
and retina, but with a timing of oscillation different from that seen in the SCN.
Sequence homology and circadian patterns of expression suggest that RIGUI is a mammalian
ortholog of the Drosophila period gene, raising the possibility that a regulator of
circadian clocks is conserved.