RRM2B and Mitochondrial myopathy: Mutations in mitochondrial genes, such as MT-ATP6 (Burrage et al, 2014), and nuclear genes, such as MGME1 (Kornblum et al, 2013) [MIM:615076] and RRM2B (Keshavan et al, 2020) [MIM:604712], which are essential for mitochondrial function, can cause mitochondrial myopathy [MIM:500009], sometimes accompanied by ptosis.