a PCR-based resequencing approach. Indeed, some protein-coding genes may not yet have been annotated and it is conceivable that some XLMR-associated genes are not protein coding. It is also possible that mental retardation in a proportion of the families in the screen may not be due to an underlying abnormality on the X chromosome. In particular, some sibling pairs could be chance clusters of sporadic mental retardation cases or be due to autosomal recessive mutations.
