To study mechanism of chromosomal translocation, we analyzed the breakpoints (b/p) of the PML and RARA genes in 120 and 5 patients with de novo and secondary (therapy-related) acute promyelocytic leukemia (APL), respectively. In de novo APL, the b/p in the PML gene were clustered in introns 3 (bcr 3; 30%) and around intron 6 (bcr 1 and 2: 70%). The b/p of the RARA gene were widely distributed in intron 2. In studied 8 de novo APL patients, no consensus sequence-motif was found around the b/p, but there were identical stretches of one to seven nucleotides between the PML and RARA genes in the joining regions, suggesting non-selective DNA double strand cleavage followed by single strand base-pairing within identical short stretches as a molecular mechanism of the translocation. In 4 secondary APL patients after chemotherapy including etoposide against Langerhans cell histiocytosis, the b/p of the PML gene were located in intron 6, and those of the RARA gene were in a restricted region within intron 2, 1 kb EcoRI-BamHI fragment, while in an APL patient after chemotherapy without etoposide against breast cancer, the b/p of the PML and RARA genes were located in intron 6 and another region within intron 2, respectively. These data suggest that a different mechanism was associated with the t(15;17) translocation in etoposide-related APL.