TK2 is associated with mitochondrial DNA depletion syndrome 2 (MTDPS2), commonly associated with myopathy with a wide clinical variability that includes intrafamilial variability where in a sibling pair, the brother had electrophysiologic findings compatible with SMA and was alive at age 4 years while his sister died at 2 years with severe weakness and hypotonia from the first months of life [48]. The gene discussed is TK2; the disease is proximal spinal muscular atrophy.