Sequence analysis of the translocation breakpoint junctions on the der(14q-) chromosome in six patients carrying a t(10;14) chromosome translocation revealed that the breakpoint occurred 5' to the HOX11 protooncogene at the breakpoint cluster region. HOX11 coding sequence was not effected. The translocation resulted in the joining of the V-(D)-J recombination signals 5' to the T-cell receptor D delta 2 segment on chromosome 14 with chromosome 10 at a location within a heptamer-like sequence. At the breakpoint junctions, the insertion of extra nucleotides, N-nucleotides, including P-nucleotides, was evident. The mechanism involved in this process is discussed.