TK2 and myopathy: However, a later report demonstrated long-term clevudine-associated myopathy was linked to mitochondrial injury by electron microscopy and mitochondrial DNA depletion (15), possibly due to exhaustion of available thymidine kinase 2 (TK2) (17), a key enzyme needed for mitochondrial DNA synthesis and utilized in the conversion of clevudine to the 5′-monophosphate.