In conclusion, the de novo novel nonsense pathogenic IGSF1 variant c.3343C > T, p.(Gln1115*) results in a severe phenotype with many of the previously described features of IGSF1 deficiency, including dysregulated GH secretion, but no hypoprolactinemia, adding to our knowledge of IGSF1 deficiency syndrome and its variable phenotype. Here, IGSF1 is linked to hyperinsulinemic hypoglycemia, familial, 4.