Six patients with de novo acute myeloid leukemia (AML) and a t(2;3)(p15-21;q26-27) were identified among approximately 1000 cases enrolled in the GIMEMA trial. The t(2;3) was the sole anomaly in three patients, whereas in three cases monosomy 7, trisomy 15 and 22, and trisomy 14 represented additional aberrations. No cryptic chromosome deletions at 5q, 7q, 12p, and 20q were observed. One patient carried a FLT3 D835 mutation; FLT3 internal tandem duplication (ITD) was not detected in three patients tested. Characterization of the translocation breakpoints using a 3q26 BAC contig specific for the PRDM3 locus showed that the breakpoints were located 5' to EVIl as follows: within myelodysplatic syndrome (MDS) intron 1 (# 3), between MDS1 exons 2 and 3 in three patients (# 1, 2, 4) with a 170bp cryptic deletion distal to the breakpoint in one (# 2), and in a more centromeric position spanning from intron 2 to the 5' region of EVI1 (# 6, 5). A set of 2p16-21 BAC probes showed that the breakpoints on chromosome 2p were located within BCL11A in two separate regions (# 1, 4 and # 2-5), within the thyroid adenoma-associated (THADA) gene (# 6) or distal to the ZFP36L2 locus (# 3). Regulatory elements were present in proximity of these breakpoints. RACE PCR studies revealed a chimeric transcript in 1/6 patient analyzed, but no fusion protein. Quantitative PCR showed a 21-58-fold over-expression of the EVIl gene in all cases analyzed. The patients showed dysplasia of at least two myeloid cell lineages in all cases; they had a low-to-normal platelet count and displayed an immature CD34+ CD117+ immunophenotype. Despite intensive chemotherapy and a median age of 43 years (range 36-59), only two patients attained a short-lived response; one patient is alive with active disease at 12 months, five died at 4-14 months. We arrived at the following conclusions: (a) the t(2;3) is a recurrent translocation having an approximate 0.5% incidence in adult AML; (b) breakpoints involve the 5' region of EVIl at 3q26, and the BCL11A, the THADA gene or other regions at 2p16.1-21; (c) cryptic deletions distal to the 3q26 breakpoint may occur in some cases; (d) the juxtaposition of the 5' region of EVIl with regulatory elements normally located on chromosome 2 brings about EVI1 overexpression; (e) clinical outcome in these cases is severe.