We report on a 13-month-old girl showing dysmorphic features and a delay in psychomotor development. She was diagnosed with a balanced de novo translocation 46,X,t(X; 13)(p 11,2; p 13) and non-random inactivation of the X chromosome. FISH analysis, employing the X chromosome centromere and XIST-region-specific probes, showed that the XIST locus was not involved in the translocation. Selective inactivation of paternal X, which was involved in translocation, was revealed by the HUMARA assay. The pattern of methylation of 5 genes located within Xp, which are normally silenced on an inactive X chromosome, corresponded to an active (unmethylated) X chromosome. These results revealed that in our proband the X chromosome involved in translocation (Xt) was preferentially inactivated. However, genes located on the translocated Xp did not include XIST. This resulted in functional Xp disomy, which most probably accounts for the abnormal phenotype in our patient.
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