MT-ND4L and MELAS syndrome: Respiratory chain complex I is the biggest and most complicated calcium-calmodulin complex in the respiratory chain, including 7 nuclear subunits (MT-ND1–6 and MT-ND4L) encoded by mtDNA and 38 nuclear subunits encoded by nDNA.[6] The defects in respiratory chain I could bring about multiple clinical diseases and MELAS syndrome is the most common 1.[7] It is usually misdiagnosed because of complex and diverse clinical features without specificity and highly heterogeneous genotype.