What the title says.
For instance in Drosophila melanogaster and Mus musculus (see Patke et al. 2020) the CLOCK, CYCLE/BMAL, CRYPTOCHROME, PERIOD setup seems to be conserved. But other components also appear to be orthologs, and can be considered conserved (e.g., VRILLE from D. melanogaster and NFIL3 from M. musculus).
So in this case, what constitutes the "core" clock?