The hypomethylation of the CpG island in the proximal promoter of the HSD17B10 gene provides additional evidence for the variable escape of the HSD17B10 gene from X-chromosome inactivation which could influence the range of severity of HSD10 deficiency, an inherited error in isoleucine metabolism, in heterozygous females. This evidence concerns the gene FSIP1 and hyperinsulinemic hypoglycemia, familial, 4.