GUCA1C encodes GCAP3 (guanylate cyclase-activating protein 3), a cone-specific calcium-binding protein that regulates photoreceptor guanylate cyclases 1. GCAP3 stimulates guanylate cyclases GC1 and GC2 when intracellular calcium is low and inhibits them when calcium is elevated 12, a mechanism critical for recovery of the dark state following light exposure 3. Unlike GCAP1, GCAP3 requires myristoylation for functional activation of GC1, and the protein naturally forms temperature-dependent dimeric aggregates 4. GCAP3 is expressed exclusively in human cone photoreceptors and ciliary epithelium 56. Disease-relevant mutations include the D101H variant associated with retinitis pigmentosa, which abolishes GC1 activation capacity 4, and homozygous nonsense variants (p.Glu18Ter) linked to primary congenital glaucoma with retinal ganglion cell apoptosis 6. Loss of GUCA1C function in knockout zebrafish induces Müller cell gliosis and glaucoma-like retinal damage, indicating roles in both phototransduction and intraocular pressure regulation 6. GUCA1C represents an important candidate gene for hereditary retinal and glaucomatous conditions.