The majority of myeloid-antigen-positive (My þ ) childhood B-cell precursor acute lymphoblastic leukaemias express TEL-AML1 fusion transcripts A. B ARUCHEL , 1,2 J. M. C AYUELA , 1 P. B ALLERINI , 3 J. L ANDMAN-PARKER , 4 V. C EZARD, 1 H. F IRAT, 3 E. H ADDAD, 5 M. F. AUCLERC , 2 F. VALENSI , 6 Y. E. C AYRE , 3 E. A. MACINTYRE 6 AND F. S IGAUX 11 Molecular Haematology Laboratory and INSERM Unit U432, 2 Paediatric Haematology Unit, Ho ˆpital Saint Louis, 3 Haematology Laboratory and INSERM Unit U417, 4 Onco-Haematology Unit, 5 Armand-Trousseau Hospital Immuno-Haematology Unit, 6 Haematology Laboratory, Ho ˆpital Necker, Paris, France Received 14 April 1997; accepted for publication 15 July 1997 Summary. The t(12;21) translocation fuses the TEL and AML1 genes and has been found in up to 28% of paediatric B-cell precursor acute lymphoblastic leukaemias (BCP-ALL). The AML1 gene is a transcription factor which regulates expression of several myeloid differentiation associated genes. A molecular analysis of TEL-AML1, E2A-PBX1, MLL-AF4, BCR-ABL expression and an immunophenotypic study of CD13/CD33 myeloid antigen expression have been performed prospectively on tumour cells from 96 paediatric BCP-ALL patients. Percentages of CD13 or CD33 expressing leukaemic cells were found to be higher in TEL-AML1 positive cases (n ¼ 22) than in TEL-AML1 negative (n ¼ 74) cases (P < 0 . 001). In 22/96 cases (23%) >10% of neoplastic cells were found to express at least one of the two markers. In 14 of these cases (63%), TEL-AML1 expression was detected, whereas t(4;11), t(11;19) and t(9;22) translocations were found by molecular methods in only three cases (14%). In four cases (18%) no molecular marker was found. These data show that TEL-AML1 expression is significantly associated with myeloid antigen expression by leukaemic cells and suggests that the prognostic significance of myeloid antigen expression in paediatric ALLs should be re-evaluated in the light of molecular cytogenetic markers. Keywords: acute lymphoblastic leukaemia, childhood, t(12;21), TEL-AML1, myeloid antigens. The t(12;21)(p13;q22) which fuses the TEL and AML1 genes (Golub et al, 1995; Romana et al, 1995a) occurs frequently in paediatric (up to 28% of cases (Cayuela et al, 1996; Raynaud et al, 1996a; Romana et al, 1995b; Shurtleff et al, 1995)) but not in adult B-cell-progenitor acute lymphoblas- tic leukaemia (BCP-ALL) (Cayuela et al, 1996; Raynaud et al, 1996b). To date, the main feature suggested to be associated with this subset is a good prognosis despite the lack of hyperdiploidy (Shurtleff et al, 1995). In a recent study of eight BCP-ALL expressing TEL-AML1 fusion transcripts, we observed that CD13 and/or CD33 myeloid associated antigens were expressed in four cases (so called My þ ALLs) (Cayuela et al, 1996). Since the AML1 gene is involved in the control of haemopoietic differentiation (Okuda et al, 1996), we analysed the correlation between myeloid antigen and TEL-AML1 expression in a series of 96 paediatric BCP-ALLs consecutively enrolled in two centres, taking advantage of the molecular screening of TEL-AML1, E2A-PBX1, BCR-ABL and ALL-AF4 fusion transcript expres- sion (corresponding to the t(12;21), t(1;19), t(9;22) and t(4;11) translocations, respectively) which was performed within the FRALLE 93 clinical trial (Baruchel et al, 1994). MATERIALS AND METHODS Patient samples. Diagnosis of ALL was made according to standard haematological and immunophenotypic criteria (Pui, 1995). Bone marrow (BM) or peripheral blood (PB) samples were obtained from 107 (including 89 B-cell precursor (BCP-ALLs) cases) at Saint-Louis Hospital (first series) and from 19 cases (including 15 BCP-ALLs) at Necker-Enfants Malades Hospital (second series). In five of the 89 BCP-ALL of the first series, TEL-AML1 expression could not be evaluated and in 3/84 remaining cases myeloid British Journal of Haematology , 1997, 99, 101–106 101  1997 Blackwell Science Ltd Correspondence: Dr F. Sigaux, Laboratory of Molecular Haematol- ogy, INSERM Unit U462, Centre Hayem, Ho ˆpital Saint-Louis, 1 av. Claude Vellefaux, 75475 Paris Cedex 10, France.