Homodimer. Component of the circadian core oscillator, which includes the CRY proteins, CLOCK or NPAS2, BMAL1 or BMAL2, CSNK1D and/or CSNK1E, TIMELESS and the PER proteins. Interacts directly with PER1, PER2, CRY1, CRY2, and TIMELESS. Interaction with CSNK1D or CSKN1E promotes nuclear location of PER proteins.