Segregation analysis on five samples of families with Hunter's syndrome (158 
cases overall) shows that the mutant allele segregates in agreement with 
Mendelian expectations for an X linked recessive disease, but the proportion of 
sporadic cases is significantly lower than expected under mutation-selection 
equilibrium. Heterogeneity among the samples is apparent, but it is caused 
entirely by a sample of Ashkenazi families, whose segregation pattern has 
previously been interpreted as supporting the hypothesis of prenatal selection 
in favour of the pathological allele. Conversely, our joint analysis of the five 
samples by a maximum likelihood approach does not suggest segregation 
distortion. Possible reasons for the apparent lack of sporadic cases include the 
effect of ascertainment bias.