Several non-random translocation breakpoints associated with leukemia or lymphoma have been shown to occur in chromosome band 11q23 between the genes CD3G and PBGD, a distance of approximately 750 kb. A combination of yeast artificial chromosome (YAC) cloning, in situ hybridization, and pulsed field gel electrophoresis (PFGE) experiments has further refined the interval containing one of these breakpoints, t(4;11)(q21;q23), to within 200 kb of CD3G. We have extended the PFGE analysis to show that the t(4;11) breakpoint lies in a region of approximately 100 kb, situated 100 kb distal to CD3G. Furthermore, we show that a second 11q23 breakpoint, t(11;14)(q23;q32), which was also previously mapped between CD3G and PBGD, is distinct from that of the t(4;11) chromosome. The 11q23 sequences that are involved at the t(11;14) breakpoint are not present in a YAC containing the t(4;11) breakpoint. The t(11;14) breakpoint has been localized on the PFGE map of the CD3G-PBGD interval and is at least 110 kb distal to the t(4;11) breakpoint, thus demonstrating heterogeneity among 11q23 breakpoints.