A four-year-old girl with typical Down's syndrome is described. She has 47,XX,+21 karyotype in skin and lymphocytes and 47,XX,+mar karyotype in some lymphocytes. Autoradiography and fluorescent analyses have failed to identify the +mar chromosome which has the appearance of a `D' chromosome but which may involve a translocation to a chromosome No. 21. However, the mechanisms of its formation and its significance are not certain.
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↵* Supported in part by Grant No. MR-0504A69 from the Division of Mental Retardation, Social and Rehabilitation Service, Department of Health, Education and Welfare, USA and by Institute of Child Health and Human Development Grant No. DO 4612, Mental Retardation Research Center, Neuropsychiatric Institute, University of California, Los Angeles, USA.
Send reprint requests to: Dr Robert S. Sparkes, Division of Medical Genetics, Department of Medicine, UCLA School of Medicine, Los Angeles, California 90024, USA.