The 8;21 chromosomal translocation. (A) A chromosome preparation of a t(8;21) patient depicting normal chromosome 8, a shorter chromosome 8 with the translocated chromosome 21 (left arrow), normal chromosome 21, and a longer chromosome 21 fused to the portion of chromosome 8 (right arrow). The image was kindly provided by Dr Janet Rowley. (B) Genomic structure of t(8;21). Depicted are the exons of both AML1 and ETO organized following the reciprocal translocation. The first 5 exons of AML1 are fused to exons 2 through 11 of ETO. P1 and P2 with the horizontal arrows denote the start sites of transcription based on the usage of the distal (P1) or proximal (P2) promoters of AML1. The reciprocal ETO-AML1 fusion shows exons 1b and 1a of ETO fused to exons 6 through 12 of AML1. Filled boxes and blank boxes indicate translated and untranslated exon sequences, respectively. (C) Protein structure of AML1-ETO. As explained in the text, the commonly known full-length AML1-ETO protein is a 752-amino acid protein. The N-terminal portion of AML1 up to its runt homology domain (RHD) is fused to most of the ETO protein. Shown are the regions of homology to the Drosophila Nervy protein (NHR1 to NLH4) and 3 proline-serine-threonine-rich (PST) regions. The vertical arrow points to the fusion junction between AML1 and ETO.