Decreasing mosaicism in Down's syndrome.

A girl with trisomy-21/normal mosaicism has been observed for approximately 10 years. Her appearance is reminiscent of, but not typical for Down's syndrome, and her intellectual abilities are impaired but not to the same severe degree expected in Down's syndrome. These observations are consistent with the belief that, in persons ascertained by clinical resemblance to Down's syndrome. The cytogenetic studies performed longitudinally demonstrated a significant decrease in the proportion of blood lymphocytes showing the trisomic line. The maximal decrease occurred in the first year of life. A direct preparation from the bone marrow and a fibroblast culture at 2.5 months showed 3 and 4% trisomic cells, respectively, corresponding to the level of trisomic cells from the blood cultures during the second year of life.