Yet, HK1 expression, which is normally disallowed in the beta cells, may cause significant fasting hyperinsulinism even in low-grade mosaic HK1-CHI, in keeping with the very high glucose affinity of hexokinase 1 (Km<0.05 mM) (34), compared to a physiological Km of 5 mM for glucokinase (hexokinase 4). The gene discussed is GCK; the disease is hyperinsulinism.