CSNK1D (casein kinase 1 delta) is a serine/threonine protein kinase essential for regulating diverse cellular processes including Wnt signaling, circadian rhythms, and DNA repair 1. As a central circadian clock component, CSNK1D regulates PER1 and PER2 phosphorylation to control circadian period length and nuclear transport 2. The kinase phosphorylates numerous substrates including tau protein, YAP1, DVL proteins, and TOP2A, modulating cell proliferation, survival, and polarity 3. Clinically, CSNK1D dysregulation associates with multiple malignancies. Elevated CSNK1D expression correlates with poor prognosis in hepatocellular carcinoma, head and neck squamous cell carcinoma, glioblastoma, and multiple myeloma, where it functions as an independent prognostic biomarker 4567. In HNSCC, CSNK1D regulates the hedgehog pathway through SHH-PTCH1 interactions 5. In multiple myeloma, CSNK1D sustains mitochondrial metabolism and oxidative phosphorylation 7. Beyond cancer, CSNK1D contributes to osteoporosis by promoting osteoclastogenesis through melatonin-related pathways 2, and to amyotrophic lateral sclerosis through TDP-43 phosphorylation-mediated neuronal toxicity 8. CSNK1D mutations cause familial advanced sleep phase syndrome, reflecting its circadian function 1. CK1δ/ε inhibitors emerge as promising therapeutic targets across these disease contexts.