Dr. Rutsch joined Dr. Terkeltaub’s lab after treating a GACI infant, and, together, they determined that the PPi deficiency in GACI patients was due to low or absent levels of ENPP1 and that the loss of ENPP1 catalytic activity could not be compensated by any of the other members of the ENPP family known at the time (10). This evidence concerns the gene ENPP1 and hyperinsulinemic hypoglycemia, familial, 4.