A balanced maternal chromosome translocation (9p24;21q214) resulted in two offspring with unbalanced karyotypes. One of these, a girl trisomic for both segment 9pter→9p24 and segment 21pter→21q214, was found to have a SOD-A activity not significantly different from those found in a group of five cases with trisomy 21. However, clinical evaluation of this girl revealed no symptoms of the Down syndrome. These findings suggest that, providing the gene dosage theory is correct, the gene for SOD-A is probably localized on chromosome 21 proximal to, or in, band
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