Nonrandom chromosome abnormalities frequently are seen in particular subtypes of human leukemia and lymphoma. These abnormalities are considered to be involved in the neoplastic transformation and in tumor progression. The translocation (8;21) (q22;q22) is consistently associated with acute myeloid leukemia with maturation (French-American-British classification subtype M2; AML-M2). It accounts for 40% of pediatric type AML-M2 Molecular cloning of the chromosome 8–21 translocation breakpoint showed clustering on chromosome 21 within a limited region in the AML1 gene and on chromosome 8 within a limited region in the ETO gene (Eight Twenty One) also called MTG8 (Myeloid Translocation Gene on chromosome 8) (1 –7 ). The t(8;21) results in a chimerical AML1/ETO gene on the der(8) chromosome. Rearrangement of AML1 was also detected in a patient with 8q- and only one chromosome 21, but without 21q+. This indicates that the molecular events on the der(8) chromosome leading to the chimerical AML1/ETO gene are more important than the events on the reciprocal 21q+ chromosome (8 ).