CYTH4 (cytohesin 4) is a guanine-nucleotide exchange factor that promotes activation of ADP-ribosylation factors (ARF1 and ARF5) through GDP-to-GTP exchange, facilitating protein trafficking and membrane dynamics 1. The gene localizes to the cytosol, Golgi membrane, and plasma membrane, regulating vesicle-mediated transport and ARF protein signaling 2. CYTH4 exhibits multiple disease associations. A primate-specific GTTT-repeat in its core promoter demonstrates protective effects against neurodegeneration; shorter alleles (GTTT)6 are significantly associated with multiple sclerosis, Alzheimer's disease, and bipolar disorder, while longer alleles confer neuroprotection 3. CYTH4 is identified as a loneliness-associated switch gene overlapping with genetic risk factors for major depressive disorder 4. In cancer contexts, CYTH4 is upregulated in glioma-associated M2 macrophages and correlates with pyroptosis and poor prognosis 2; it is similarly elevated in ovarian cancer with worse survival outcomes and altered immune infiltration 1. A genome-wide association study identified CYTH4 as a fibroid risk locus in African American women, where lower predicted expression correlates with increased risk 5. Recent studies identify CYTH4 as a potential biomarker in Alzheimer's disease and mild cognitive impairment 6, and as a predictor of immunotherapy response in melanoma 7.