Molecular detection of altered X-inactivation patterns in the diagnosis of genetic disease.

It is widely assumed that when a female carrier of a genetic disorder exhibits clinical signs of the disorder it is due to chance non-random X-inactivation in particular tissues. Recently molecular methods have become available for the analysis of X-chromosome inactivation status. These are based either on the methylation patterns of DNA from the active and inactive chromosomes or on the rescue of active X chromosomes in somatic cell hybrids. As a consequence of the molecular studies, it has become obvious that there are some special cases of non-random X-inactivation patterns. These include females carrying X-linked immunodeficiencies and, sometimes, one of a pair of identical female twins.