Trisomy 21 as an acquired clonal chromosome change has been described in 642 of the 10,625 human neoplasms with chromosome aberrations known from the cytogenetic literature. A total of 590 of the 642 cases (92%) are hematologic disorders and malignant lymphomas. The incidence of trisomy 21 is similar (4.1%–6.7%) in acute myeloid leukemia, myeloproliferative disorders, myelodysplastic syndromes, chronic lymphoproliferative disorders, and malignant lymphomas; it is substantially higher (14.8%) in acute lymphocytic leukemia (ALL). In most cases, the extra chromosome 21 is present together with other numerical and/or structural changes. Acquired trisomy 21 is the only karyotypic abnormality in only 0.4%. Trisomy 21 has never been reported as the sole anomaly in a solid tumor.

The cytogenetic literature contains information on 62 patients with constitutional trisomy 21 and a malignant disorder in which the tumor cells have been analyzed by banding techniques. Thirty‐four of the 62 patients had AML, 16 had ALL, and 2 had acute undifferentiated leukemia. The 52 leukemic Down syndrome (DS) cases account for 1.4% of the total acute leukemias, an overrepresentation that parallels the generally increased risk of leukemia development in DS. Sixty three percent of the ALL patients and 79% of those with AML had additional changes superimposed on constitutional trisomy 21. These included several of the characteristic primary leukemia‐associated aberrations: 5q−, 7q−, +8, and t(8;21) in AML, and t(1;19), t(4;11), 6q−, and 14q+ in ALL. Thus, it seems that the pattern of acquired karyotypic changes is similar in patients with DS and in individuals with a normal constitutional karyotype.