TPI1 and hyperinsulinemic hypoglycemia, familial, 4: As aforementioned, the frequency of TPI null alleles is much higher as the rare incidence of TPI deficiency [5]–[9], and in vitro measurements of a mutant TPI variant purified from E. coli demonstrated that the purified TPI protein carrying the Phe240Leu mutation exhibited a 6-fold higher activity than expected from the measurements performed with the patient erythrocyte extract [40].