Objective:To summarize the clinical characteristics of children carrying the m.8344A>G variant of MT-TK gene.Methods:A case series study was conducted to retrospectively collect data of 22 children with mitochondrial disease caused by MT-TK gene m.8344A>G variation who were treated at the Department of Neurology of Beijing Children′s Hospital of Capital Medical University from January 2012 to January 2024. Their clinical data, laboratory tests, muscle pathology, genetic testing, and the follow-up results were analyzed. Pearson correlation analysis was used for correlation analysis.Results:Among the 22 children, there were 13 boys and 9 girls. The age of onset was 5.00 (2.75, 9.00) years. Fifteen children had myoclonic epilepsy with ragged-red fibers (MERRF), 3 had Leigh syndrome (LS), and 4 had LS-MERRF overlap syndrome (LS-MERRF). Myoclonus presented and worsened progressively in all 15 MERRF children, with 10 as the initial symptom and 5 developing progressively during the disease course. Myoclonus was predominantly focal, worsening with fine motor tasks or stress. Electroencephalogram monitoring in the 15 MERRF children revealed myoclonic seizures in 10 children, with 6 classified as myoclonic epilepsy, and 4 as subcortical myoclonus. Two children had generalized myoclonic seizures, and 1 each had absence seizures and generalized seizures. Twelve children had cerebellar ataxia, 10 children exhibited exercise intolerance, and 8 children had muscle weakness. Magnetic resonance imaging (MRI) revealed periventricular white matter involvement in 1 child and bilateral hippocampal involvement in 1 child, likely due to frequent seizures. All 3 children with LS exhibited developmental regressions, accompanied with 2 symptoms include cerebellar ataxia, muscle weakness, and dysphagia. The clinical manifestations of 4 LS-MERRF overlap children presented with combined features of MERRF and LS. Cranial MRI in the 7 LS and LS-MERRF children showed brainstem involvement (all affecting the midbrain) in 6 children and basal ganglia involvement in 4 children. Among the 22 children, 12 had m.8344A>G variant levels >90%, 3 had >80%-90%, 4 had >70%-80%, and 3 had >60%-70%. Higher variant level correlated with the LS phenotype and earlier onset age ( r=0.47, -0.50; P=0.018 and 0.029, respectively). Sanger sequencing in 19 mothers revealed m.8344A>G variations in 18, with 4 showing exercise intolerance. Follow-up of 13 children on antimyoclonic treatment showed>75% reduction in seizures with levetiracetam monotherapy in 2 children, with combination therapy required in others. Most achieved >50% seizures reduction within 2 years, but the effectiveness declined with disease progression. Conclusions:The m.8344A>G variant is rare, with MERRF being the most common phenotype, while LS and LS-MERRF are less common. Children with higher ratio of the m.8344A>G variant are more likely to present LS phenotype. Myoclonus, primarily focal, is a key feature, with levetiracetam as the first-line treatment and benzodiazepines recommended for refractory cases.