We have analyzed the type of MYC/IG heavy-chain locus (IGH) rearrangement present in 15 patients affected by t(8;14)-positive primary Burkitt's lymphoma or acute lymphoblastic leukemia of the L3 type in an attempt to map in detail the locations of the chromosome 8 and chromosome 14 breakpoints. The almost constant position of the chromosome 8 breakpoint (within or immediately 5' of the MYC gene) together with two distinct clusters of breakpoints on chromosome 14 resulted in two main types of MYC/IGH (present in 12 of 15 cases). In the first type (six cases), the MYC gene or at least its coding portion was joined with the JH region on chromosome 14, whereas in the second, present in another six cases, the MYC gene and the C alpha I region were juxtaposed. Physical linkage between the translocated MYC and a known enhancer element of the IGH locus is the common feature in the two types of rearrangement, suggesting that a high-level constitutive expression plays a prominent role in MYC activation. Interestingly, the chromosome 14 break site within the switch alpha 1 region, which has been only occasionally described in other cases, is present in 40% of our patients, suggesting the existence of preferential breakpoint cluster regions in cases of similar geographic origin.