Mutations in most of more than 20 known genes causing nonspecific form of X-linked MR (MRX) are very rare and may account for less than 0.5-1% of MR. Linkage studies in extended pedigrees followed by mutational analysis of known MRX genes in the linked interval are often the only way to identify a genetic cause of the disorder.
The 13 MRX genes identified to date account for less than one-fifth of all MRX, suggesting that numerous gene defects cause the disorder in other families.
On the other hand, given the considerable genetic heterogeneity in MRX, one should be extremely cautious in using interfamilial linkage data to narrow down the localisation of MRX genes.