Sanfilippo B syndrome (MPS III B): mild and severe forms within the same sibship.

Clinical heterogeneity for Sanfilippo B syndrome (MPS III B) in the same family has never been reported previously. We describe two clinically severe cases and one clinically mild case of MPS III B in a Neapolitan sibship. We could not detect N-acetyl-alpha-D-glucosaminidase activity in the sera of either the severe or mild cases. Mucopolysacchariduria mainly due to heparan sulfate excretion was consistently high in the severely affected patients and extremely variable in the mildly affected one.