Untypically mild phenotype of a patient suffering from Sanfilippo syndrome B with the c.638C>T/c.889C>T (p.Pro213Leu/p.Arg297Ter) mutations in the NAGLU gene.

We suggest that the mild phenotype might arise from the partially preserved function of the mutant enzyme (p.Pro213Leu), suggesting the genotype-phenotype correlation in this case.