Intrafamilial variability in Hurler syndrome and Sanfilippo syndrome type A: implications for evaluation of new therapies.

Intrafamilial variability has not been reported previously in Hurler syndrome or Sanfilippo syndrome type A. We describe two families in which sibs with comparable deficiencies of alpha-iduronidase (Hurler) or sulfamidase (Sanfilippo type A) activities in vitro nonetheless have divergence in clinical severity and disease progression. These cases underscore the need for caution in counseling as well as the limitations of using sibs as controls in evaluating the outcome of treatment.