A t(X;14)(q28;q11) translocation was present for many years in T cells in two patients with ataxia telangiectasia (A-T), who subsequently developed T-prolymphocytic leukemia. We describe here the relationship between the translocation breakpoints in these patients with respect to two recently described genes, c6.1A and c6.1B, on Xq28 which are transcribed in opposite directions from the same CpG island. In our first patient, the Xq28 breakpoint disrupts the c6.1A gene which is consequently transcribed as a fusion mRNA with the TCR C alpha chain gene. In the second case, the Xq28 breakpoint lies within the adjacent gene c6.1B, and c6.1A is not transcribed. We show that the c6.1B gene is transcribed in both of our patients. c6.1B may be important in the initial clonal proliferation of T lymphocytes which commonly precedes transformation to T-PLL in ataxia telangiectasia patients. The same gene may also be involved in the development of T-PLL in the non-A-T population.