POLG2 and hyperinsulinemic hypoglycemia, familial, 4: This mosaic pattern of staining across cells has been previously observed in primary mitochondrial disease patients with combined OXPHOS deficiency due to an underlying molecular etiology that results in a variable mutational load in mtDNA across patient cells.10 To our knowledge this is the first demonstration of this staining pattern in cells from patients with a POLG2 pathogenic variant as well as the first documentation of this pattern in any patient with multiple mtDNA deletions.