Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. It can phosphorylate a large number of proteins. Participates in Wnt signaling. Phosphorylates CTNNB1 at 'Ser-45'. May phosphorylate PER1 and PER2. May play a role in segregating chromosomes during mitosis. {ECO:0000269|PubMed:11955436; ECO:0000269|PubMed:1409656; ECO:0000269|PubMed:18305108}.