The core transcriptional circadian clock of (A) H. sapiens and (B) O. tauri . CK1 and CK2 regulate both clocks though PER and CCA1 phosphorylation. The downstream core clock however differs: in H. sapiens, CK2 has been shown to stabilise PER2 and promote its nuclear accumulation . PER2 in turn stabilises Cryptochrome (CRY)1/2 in complex, which represses the transcriptional activation of PER1/2/3 via CLOCK/BMAL1/2, creating an oscillating feedback loop. In O. tauri, CK2 phosphorylates the morning-expressed CCA1 (by similarity with A. thaliana, which represses the transcription of the evening-expressed TOC1. TOC1 in return represses the transcription of CCA1, again creating an oscillating feedback loop. PAS – Period – FAD – HLH – helix-loop-helix domain MYB – RRR – response regulator receiver domain CCT – CCT motif.