Essential serine/threonine-protein kinase that regulates diverse cellular growth and survival processes including Wnt signaling, DNA repair and circadian rhythms. It can phosphorylate a large number of proteins. Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. Phosphorylates connexin-43/GJA1, MAP1A, SNAPIN, MAPT/TAU, TOP2A, DCK, HIF1A, EIF6, p53/TP53, DVL2, DVL3, ESR1, AIB1/NCOA3, DNMT1, PKD2, YAP1, PER1 and PER2. Central component of the circadian clock. In balance with PP1, determines the circadian period length through the regulation of the speed and rhythmicity of PER1 and PER2 phosphorylation. Controls PER1 and PER2 nuclear transport and degradation. YAP1 phosphorylation promotes its SCF(beta-TRCP) E3 ubiquitin ligase-mediated ubiquitination and subsequent degradation. DNMT1 phosphorylation reduces its DNA-binding activity. Phosphorylation of ESR1 and AIB1/NCOA3 stimulates their activity and coactivation. Phosphorylation of DVL2 and DVL3 regulates WNT3A signaling pathway that controls neurite outgrowth. Phosphorylates NEDD9/HEF1 (By similarity). EIF6 phosphorylation promotes its nuclear export. Triggers down- regulation of dopamine receptors in the forebrain. Activates DCK in vitro by phosphorylation. TOP2A phosphorylation favors DNA cleavable complex formation. May regulate the formation of the mitotic spindle apparatus in extravillous trophoblast. Modulates connexin-43/GJA1 gap junction assembly by phosphorylation. Probably involved in lymphocyte physiology. Regulates fast synaptic transmission mediated by glutamate. {ECO:0000250|UniProtKB:Q9DC28, ECO:0000269|PubMed:10606744, ECO:0000269|PubMed:12270943, ECO:0000269|PubMed:14761950, ECO:0000269|PubMed:16027726, ECO:0000269|PubMed:17562708, ECO:0000269|PubMed:17962809, ECO:0000269|PubMed:19043076, ECO:0000269|PubMed:20041275, ECO:0000269|PubMed:20048001, ECO:0000269|PubMed:20407760, ECO:0000269|PubMed:20637175, ECO:0000269|PubMed:20696890, ECO:0000269|PubMed:20699359, ECO:0000269|PubMed:21084295, ECO:0000269|PubMed:21422228, ECO:0000269|PubMed:23636092}. This is the function of Ensembl gene identifier ENSG00000141551 (CSNK1D, casein kinase 1 delta).