3 PATIENTS WITH RING (X)-CHROMOSOMES AND A SEVERE PHENOTYPE

Three patients with mosaicism and a cell line containing a small ring (X) chromosome are described. Their phenotype is similar to several pr eviously reported patients with a 45,X/46,X,r(X) karyotype and a pheno type far more severely affected than expected in Turner's syndrome. Th e clinical picture includes mental retardation, a facial appearance re miniscent of the Kabuki make up syndrome, and limb anomalies. Some of the patients also had streaky hyperpigmentation of the skin in a patte rn suggesting dermal mosaicism. It has been hypothesised that the seve re phenotype might be the result of the small r(X) chromosome remainin g active. However, there is little critical evidence to support this s uggestion, while there is considerable evidence against it, including (1) a similar phenotype in 45,X/46,X,r(Y) patients, (2) the late repli cation of some of the small r(X) chromosomes associated with this phen otype, and (3) the expression of XIST in some of the affected patients .