The uptake of rhGAA is defective in fibroblasts from other lysosomal storage disorders. (A) Human control fibroblasts and fibroblasts from MPSII and MPSVI patients and (B) mouse embryonic fibroblasts (MEF) from control mice and Sumf1-/- mice were incubated with rhGAA (50 μg/ml) for 24 hours. GAA activity is expressed as nmol 4-methylumbelliferone liberated/mg protein/h. Activities are expressed as the difference between the activity after 24 hours of incubation and baseline activity. All mutant cell lines analyzed show reduced GAA activity as compared with control cells.