X-linked mental retardation with macro-orchidism and marker-X chromosomes

The family of 2 men with X-linked mental retardation was investigated for the presence of the marker-X chromosome and macro-orchidism. Lymphocyte cultures were set up in media F-10 and 199. Chromosomes were G-band-stained and slides coded for blind analysis. Marker-X chromosomes and autosomal gaps were seen only in cultures prepared with medium 199. Autosomal gaps were seen in all family members and controls. The marker-X chromosome was present in affected male (3% and 15.5%) and female family members (0.5%–6.5%) but not in controls or a normal male family member. Bilateral macro-orchidism was present in 1 affected male and unilateral macro-orchidism in the other. Both had normal penile measurements. The replication pattern of the marker-X chromosome was studied in 1 woman by BrdU labelling. BrdU decreases the frequency of marker-X expression. As the marker-X chromosome was the late-replicating X in 9/20 cells, it appears that there is no preferential inactivation of the marker-X in this woman.