We utilized LDI-PCR to clone the translocation breakpoint. Genomic DNA from leukemic cells was digested with HindIII, ligated at low concentration, and subjected to two rounds of PCR using nested primers within exon 5 of MLL (first round: 50-TCCAGGAAGTCAAGCAAGCAGGTC-30, 50-GGAGTGGTGGC CTGTTTGGATTCA-30; second round: 50-GCCTCAGCCACCTACTACAGGAC-30, 50-CTTTCGTGGAGGAGGCTCACTAC-30). LDI-PCR performed on the leukemic DNA sample yielded a 6.8 kb band, the same size as the rearranged band detected by Southern analysis. Partial sequencing and basic local alignment search tool (BLAST) analysis7 of this band identified 122bp of unknown sequence immediately downstream of the 50 portion of MLL intron 6 (Figure 2a), which was 100% identical to intronic sequence within the KIAA1676 gene on chromosome 10q22. Based on its involvement in at (10; 11)-associated leukemia, we have designated this gene Ten-Eleven Translocation-1 (TET1). Just prior to submission of this manuscript, TET1 was independently identified as the partner gene in another case of AML containing t (10; 11)(q22; q23). 5 In our case, the breakpoint within MLL intron 6 was 20 bp upstream of that reported by Ono et al, 5 whereas the breakpoint within TET1 intron 8 was 950 bp downstream of that reported by these investigators. To independently confirm that TET1 was the translocation partner, RT-PCR analysis was performed on total RNA extracted from either leukemic cells or control peripheral blood leukocytes (PBL) using 50 and 30 primers derived from MLL exon 5 (MLL 50-GCCTCAGCCACCTACTACAGGAC-30) and TET1 exon 9 (TET1 50-GGAGCTGCTCATC-TTGAGGAATAAC-30), respectively (Figure 2b). RT-PCR performed on the leukemic sample, but not the control, generated a band of the predicted size. Expression of the reciprocal TET1-MLL transcript was not detected in RT-PCR analyses using two separate primer pairs (data not shown). Sequencing of the cloned RT-PCR product revealed, as predicted, a chimeric transcript consisting of MLL exon 6 fused in-frame to TET1 exon 9 (Figure 2c). TET1 is predicted to encode a protein of 2136 amino acids with a molecular mass of 235.3 kDa. TET1 contains a CXXC domain at position 583–624, a coiled-coil region near the C-terminus (position 2062–2091), and three candidate bipartite nuclear localization signals. 5 The MLL-TET1 fusion protein is predicted to have a molecular mass of 204.4 kDa and to retain the AT hooks, subnuclear localization domains, and the CXXC motif of MLL, as well as the coiled-coil region and a b c d