A left shift of the oxygen dissociation curve or when venous P50 < 24 mm Hg, is a good screening test for high oxygen affinity Hgb variants, but can also indicate 2,3-bisphosphoglycerate deficiency, methemoglobinemia or PIEZO1 variants. The gene discussed is PIEZO1; the disease is hyperinsulinemic hypoglycemia, familial, 4.