A 3 1/2-year-old female with clinical features of Down's syndrome was found to have extra chromosome material on the long arm of one of the X chromosomes, 46,XXq+. The parental karyotypes were normal. In the light of the clinical features of the proband an the banding characteristics of the extra chromosome material, the patient was thought to have a de novo (X;21) translocation. The results of late replication studies with BUdR and enzyme superoxide dismutase (SOD) assays in the proband suggest that: (1) the presumed (X;21) translocation chromosome was the late replicating chromosome; (2) the spread of inactivation extended from the Xq segment of the translocation chromosome to the proximal part of the segment derived from chromosome 21, leading to the inactivation of the autosomal gene for enzyme SOD; (3) the remaining distal portion of the (X;21) translocation chromosome, a part of a segment presumably derived from chromosome 21, was spared from the spread of inactivation so that this part was still genetically active and responsible for the Down's phenotype; (4) therefore, the main determinants for a Down's phenotype may be located more distally (q22.2 or q22.3 or both) than the SOD gene (q22.1) on the long arm of chromosome 21.
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