COQ5 encodes a methyltransferase essential for coenzyme Q10 (CoQ10) biosynthesis, catalyzing the conversion of 2-decaprenyl-6-methoxy-1,4-benzoquinol to 2-decaprenyl-3-methyl-6-methoxy-1,4-benzoquinol 1. The protein localizes to mitochondria, where the mature form is processed from a precursor, and its expression is regulated by cellular energy status 2. COQ5 functions as part of the electron transport chain machinery and is transcriptionally controlled by multiple factors including carbon source availability 3. Biallelic mutations in COQ5 cause primary coenzyme Q10 deficiency, presenting with a core phenotype of intellectual disability, encephalopathy, cerebellar ataxia and atrophy, speech regression, short stature, and developmental delays 4. Additional features may include microcephaly, dysmorphic features, and social regression 4. The molecular mechanism underlying COQ5-linked pathology involves mRNA mis-splicing leading to reduced functional protein and consequent CoQ10 deficiency 4. COQ5 variants have also been associated with retinitis pigmentosa, expanding the clinical spectrum beyond neurological manifestations 5. The gene's role in mitochondrial function makes it relevant to various cellular processes including oxidative metabolism and quality control mechanisms 6.