Several genotype/phenotype correlation studies in GSDII patients have well established that a) GAA residual activity of less than 3% the normal is highly recurrent in the classic infantile form whereas it neither falls below this threshold nor exceeds 35% of control values in late-onset patients; b) certain combinations of GAA mutations invariably cause the classic infantile form and others the less severe ones [1]; c) GAA mutations modify the levels of GAA residual activity in a constant and reproducible way when assessed in vitro, but their expression is less well predictable in vivo [1]. The gene discussed is GAA; the disease is glycogen storage disease II.