A subset of genes located on chromosome 21. The distal region of chromosome 21 present in 3 copies in cells from children with DS contains several putative leukemogenic genes including RUNX1. In addition, the region 21q22.3 contains the gene that encodes the enzyme cystathionine β-synthase (CBS), which is involved in the metabolism of cytosine arabinoside and may be responsible for increased sensitivity of DS AMKL blasts to this compound.⁹¹