Mental retardation in association with a balanced X-autosome translocation and random inactivation of the X chromosomes

A woman whose karyotype shows an apparently balanced reciprocal translocation, 46,X, t(Xq +; 10q —) is described. She is profoundly mentally retarded and shows minor physical abnormalities with normal sexual development. There is a random pattern of late replication of the normal X and the X involved in the translocation, whereas in most balanced X-autosome translocations there is preferential inactivation of the normal X.