The nature of mutant acid alpha-glucosidase (AAG) in muscle was studied in 6 
patients with Pompe disease, consisting of 2 each of the infantile, childhood 
and adult types. Anti-human liver AAG rabbit antibody prepared in the present 
study was confirmed to be monospecific by immunodiffusion, immunotitration and 
immunohistochemical methods. It was found by the immunodiffusion and enzyme 
immunoassay methods using this antibody that the mutation produced a normal 
amount of enzyme protein but the latter was an inactive form, suggesting 
structural gene mutation in 5 of the 6 cases. In the remaining childhood type 
case there was no detectable amount of enzyme protein, suggesting that the 
mutation causes a reduction in the amount of the enzyme protein or synthesis of 
unstable enzyme protein. Similarly, the enzyme activity of AAG was markedly 
reduced in all patients, but that of neutral alpha-glucosidase was the least 
reduced in the adult type, medium in the childhood type, and the most reduced in 
the infantile type.