For example, patients homozygous for a mutation in the promoter of PMM2 do not get PMM2-CDG or even a mild form of PMM2-CDG, but instead have hyperinsulinemic hypoglycemia and polycystic kidney disease because this mutation impairs binding by a kidney- and pancreas-specific transcription factor to a chromatin loop in the promoter of PMM2 (Cabezas et al., 2017). This evidence concerns the gene PMM2 and hyperinsulinemic hypoglycemia.