Inclusion Criteria:  Patients suspected of a mitochondrial disease with muscular signs (clinical, histological or biochemical) Patients with negative mtDNA and WES NGS in trio Patients with routine muscle and skin biopsies available Blood samples from parents and / or relatives available for segregation studies Informed consent of the study signed by the patient or the legal representatives of the minor patient or under guardianship Patients affiliated to social security  Exclusion Criteria:  Patients with suspected mitochondrial disease without muscle involvement Patients for whom the mtDNA NGS and WES have not been performed Patients with suspected mitochondrial disease with causal variant identified Refusal to sign the informed consent for the study Insufficient amount of frozen material or culture failure for fibroblasts