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We describe a 13-year-old boy with developmental delay and proximal muscle weakness who has monosomy 20 mosaicism in blood and skin cells. Because of asymmetric features (difference in foot size, slightly asymmetric intergluteal cleft), we performed extensive cytogenetic studies in peripheral blood and skin. In cultured and uncultured blood lymphocytes, we found 0.9 and 6.5% of cells with monosomy 20, respectively. In addition, 3.3% of uncultured skin fibroblasts and 1.5% of buccal mucosa cells had monosomy 20. This is the fifth patient published with this chromosomal condition. These patients show variable clinical features, ranging from normal to delayed motor and speech development. There is no apparent relation between the percentage of monosomic cells as studied in blood and the severity of the phenotype. This could be due to different degrees of mosaicism in the other tissues and organs, which may vary considerably from patient to patient. The degree of monosomy 20 mosaicism in blood is in most patients below the detection limit of microarray technology. Therefore, this work illustrates the necessity of detailed cytogenetic investigation of multiple cell types in developmentally retarded patients with normal microarray results, especially when there are subtle physical indications of chromosomal mosaicism.

Monosomy 20 Mosaicism Revealed by Extensive Karyotyping in Blood and Skin Cells: Case Report and Review of the Literature