GAMT and hyperinsulinemic hypoglycemia, familial, 4: In summary, the key factors facilitating GAA accumulation in the brains of patients with GAMT deficiency could be (i) the lack of efflux transport across the BBB, (ii) the increase of CRT-mediated blood-to-brain GAA transport at the BBB, and (iii) the Na+-dependent concentrative GAA transport by brain parenchymal cells.