TK2 and Mitochondrial myopathy: This pattern allows: (1) to distinguish TK2d from other mitochondrial myopathies and, therefore, to guide genetic testing, (2) to support the diagnosis when the identified variants in the TK2 gene are classified as of uncertain clinical significance, and (3) to differentiate TK2d from other myopathies with similar clinical manifestations, such as FSHD and OPMD, which is particularly important now because genetic studies are usually prioritized over muscle biopsies during the diagnostic process.