We previously demonstrated that the 11q23 breakpoint region, designated the RCK locus, of the RC-K8 B-lymphoma cell line with t(11;14)(q23;q32) is centromeric to PBGD, while breakpoints of infantile leukemia cell lines with t(11;19)(q23;p13) are detectable by pulsed-field gel electrophoresis with the CD3D probe. In the present study, using a probe within 1.0 kilobase of the t(11;14) breakpoint, we isolated a partial complementary DNA clone for the putative RCK gene, which detects a 7.5-kilobase mRNA. Sequence analysis predicted a novel protein of 472 amino acids which demonstrated sequence homology to a translation initiation factor/helicase family. We also isolated a phage clone from the CD3D/G yeast artificial chromosome clone (yB22B2) which detects 11- and 12-kilobase mRNAs, most likely for the MLL/ALL-1 gene associated t(4;11)(q21;q23) and t(11;19)(q23;p13) translocations. By pulsed-field gel electrophoresis after NotI digestion, this recombinant clone is on a 96-kilobase fragment, while RCK and PBGD probes are on a more telomeric 690-kilobase NotI fragment. These results, altogether, suggested that two different genes, RCK and MLL/ALL-1, are associated with 11q23 translocation of hematopoietic tumors.