Most commonly the genetic defect is in the GTP cyclohydrolase I gene (GCHI deficiency; Ichinose et al. 1994), the product of which supports tyrosine hydroxylase (TH) function (for TH function see Fig. 1), but the deficiency can also be attributed to other genes such as the TH gene itself (TH deficiency; Lüdecke et al. 1995). This evidence concerns the gene TH and hyperinsulinemic hypoglycemia, familial, 4.