As routinely performed to diagnose Hex deficiency in Tay-Sachs (HEXA−/−) and Sandhoff (HEXB−/−) disease (56), Hex activity can be readily measured in blood plasma using fluorescent substrates, that is, MUG (measuring total Hex activity) and MUGS (measuring HEXA-specific activity). The gene discussed is HEXA; the disease is hyperinsulinemic hypoglycemia, familial, 4.