Coenzyme Q₁₀ (CoQ₁₀) or ubiquinone is one of the two electron carriers in the mitochondrial respiratory chain which has an essential role in the process of oxidative phosphorylation. Defects in CoQ₁₀ synthesis are usually associated with the impaired function of CoQ₁₀–dependent complexes I, II and III. The recessively transmitted CoQ₁₀ deficiency has been associated with a number of phenotypically and genetically heterogeneous groups of disorders manifesting at variable age of onset. The infantile, multisystemic presentation is usually caused by mutations in genes directly involved in CoQ₁₀ biosynthesis. To date, mutations in COQ1 ( PDSS1 and PDSS2 ), COQ2 , COQ4 , COQ6 , COQ7 , COQ8A / ADCK3 , COQ8B/ADCK4 , and COQ9 genes have been identified in patients with primary form of CoQ₁₀ deficiency. Here we report novel mutations in the COQ4 gene, which were identified in an infant with profound mitochondrial disease presenting with perinatal seizures, hypertrophic cardiomyopathy and severe muscle CoQ₁₀ deficiency.