MT-TK: MT-TK is a mitochondrial transfer RNA (tRNA) gene that encodes mitochondrial transfer RNA lysine [PMC4280940]. Mutations in MT-TK have been reported in MERRF/MELAS and MERRF/Kearns-Sayre syndrome overlap syndromes [PMC9036286]. In a study, various mutations were found in different mitochondrial tRNA genes, including MT-TK and MT-TL1 [PMC7755120]. Among these mutations, the m.8344A>G mutation in the MT-TK gene is the most common [PMC4280940]. This mutation affects the function of mitochondrial tRNA lysine and has been associated with various clinical manifestations, including MERRF (Myoclonic Epilepsy with Ragged Red Fibers) and MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) syndromes [PMC4280940]. The overlap syndromes of MERRF/MELAS and MERRF/Kearns-Sayre syndrome are characterized by a combination of symptoms from both syndromes [PMC9036286]. These mutations in MT-TK can lead to impaired mitochondrial function and energy production, resulting in the clinical manifestations observed in these syndromes [PMC9036286][PMC4280940][PMC7755120].