X-cess of variants in XLMR

From a News and Views article in Nature Genetics, following up the recently mentioned sequencing effort for X-linked mental retardation:

"This study echoes old lessons from mendelian genetics that should be heard by other groups currently engaged in such large-scale sequencing surveys for discovering genes associated with human diseases. It is clear from the current work that a major challenge for this era of resequencing studies is discerning causative variation, as these will be accompanied by many other changes that each look as if they might perturb gene function. In the absence of clear functional criteria, compelling evidence of association is required to make the case that alleles are causative. As it is unlikely that there will be any simple single solution to providing functional data for all the discovered variation, the process of assigning phenotypes to newly discovered genotypes will get harder before it gets easier."

Nature Genetics: X-cess of variants in XLMR