Schematic representation of the TEL/AML1 fusion formed by the t(12; 21). The TEL gene, located at chromosome 12, band p13 (Chr. 12), encodes a protein containing helix-loop-helix (HLH) and DNA-binding (DB) domains. AML1, located on chromosome 21 (Chr. 21), encodes a protein that contains a transactivation domain (TA) and a runt homology domain (RHD). Sac I (S) and BamHI (B) sites, as well as probe pSB, are shown in TEL. The der(21) product fuses the 5′ portion of TEL, containing the helix-loop-helix domain, to nearly all of AML1. Arrows indicate the t(12; 21) breakpoints in each gene and the site of fusion.