They are due to mitochondrial dysfunction such as deletions in mitochondrial DNA (Pearson marrow-pancreas syndrome (PMPS) [23]) or mutations in nuclear genes coding for mitochondrial proteins (TRNT1: SFID form; LARS2: LARS2 deficiency form; YARS2: MLASA2 form; PUS1: MLASA1 form), or mutations in mitochondrial respiratory chain proteins (MT-ATP6, SLC19A2, NDUFB11) and mutations in the Fe-S cluster biogenesis protein (ABCB7: XLSA/A form) [18,20]. The gene discussed is PUS1; the disease is Pearson syndrome.