The rhombotin 2 gene was identified by its association with a recurrent chromosome translocation (11;14)(p13;q11), occurring in T‐cell acute lymphoblastic leukaemias (ALLs). High levels of RBTN2 have been found in T‐cell leukaemias carrying the translocation and in some T‐cell ALLs that lack, by cytogenetic and molecular techniques, translocations involving 11p13. In normal murine tissues RBTN2 has been found to be widely expressed, with high levels present in brain and early B cells. Studies carried out in mice lacking RBTN2 have demonstrated the importance of this gene in erythropoiesis.We have investigated the expression of RBTN2 in human leukaemic cells, and in human and murine normal myeloid progenitor cells. RBTN2 is expressed in the leukaemic cells of patients with pre‐B ALL, T‐ALL and acute myeloblastic leukaemia (AML). By cytogenetic and molecular techniques no abnormalities were noted in 11p13. Using clonogenic assays and single cell PCR we found that RBTN2 is expressed strongly in the precursors of mixed erythrocyte/macrophage/mast, erythrocyte, megakaryocyte, neutrophil and macrophage colonies. In contrast, RBTN2 message was low to undetectable in the mature progeny. These findings indicate that RBTN2 is expressed in leukaemias of both the myeloid and lymphoid lineages. Further, these studies show that in normal myeloid and lymphoid cells the expression of RBTN2 is present in progenitor cells and is lost as the cells terminally differentiate. This latter finding further illustrates that the detection of a RNA in a population of cells should not be interpreted to mean that all of the cells in that population express the RNA.