CLOCK (circadian locomotor output cycles kaput) is a core transcriptional regulator of the mammalian circadian clock that generates ~24-hour rhythms in gene expression controlling metabolism, sleep, body temperature, and cardiovascular function 1. As a heterodimeric partner with BMAL1, CLOCK activates transcription of clock genes (PER, CRY) and metabolic genes by binding E-box elements in promoters 2. CLOCK possesses intrinsic histone acetyltransferase activity enabling chr4 remodeling and rhythmic regulation of metabolic enzymes [PMID:28985504 via UniProt]. Circadian disruption through altered CLOCK expression contributes to metabolic pathologies including obesity, type 2 diabetes, and cardiovascular disease 1. In cancer, CLOCK amplification (5% of glioblastomas) enhances glioma stem cell self-renewal and recruits immunosuppressive microglia via OLFML3 upregulation, promoting tumor growth 3. In hepatocellular carcinoma, oncogenic signaling phosphorylates CLOCK at S106, causing nuclear export and cytosolic acetylation of PRPS to promote nucleotide synthesis and proliferation 4. CLOCK variants are associated with psychiatric disorders including depression, bipolar disorder, and schizophrenia 5. Human-specific neocortical CLOCK expression enhances cognitive flexibility through increased dendritic complexity and synaptic connectivity 6.