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Summary. The Ikaros gene is an essential regulator in development and haematopoiesis. Dysregulated Ikaros gene expression participates in leukaemic processes, as evidenced in animal models, and by analyses of blast-cell populations from leukaemic patients. We used real-time quantitative polymerase chain reaction (PCR) to evaluate the relative abundance of several Ikaros transcript isoforms in a variety of leukaemic-cell samples. Total RNA was isolated from bone-marrow or blood-cell samples collected at diagnosis in children or adult patients, 18 of whom had acute myeloblastic leukaemia (AML), 61 of whom had acute lymphoblastic leukaemia (ALL) and 11 of whom had chronic myeloid leukaemia (CML). The ratio (Ik1 1 Ik2)/ (Ik1 1 Ik2 1 Ik4 1 Ik7 1 Ik8) ranged from 13´5% to 85% and was lower (P , 0´05) in samples from patients with m-bcr-abl ALL. An alternative splicing resulting in the deletion of 30 nucleotides at the end of exon 6 was observed in leukaemic samples, and in normal thymus and bone marrow. Our results are consistent with previous reports and suggest that the pattern of expression of the different human Ikaros isoforms are not homogeneous among different subsets of leukaemias.

A subset of mobilized CD34+ cells present in patient aphereses expresses Thy1 (CDw90). This population contains most long-term culture initiating cells, as assayed with a murine stromal cell line. It also contains a significant proportion of colony-forming unit granulocyte macrophage, but very few burst-forming unit erythroid. The limited differentiation towards the erythroid lineage is further confirmed by the absence of GATA-1 mRNA in the CD34+/Thy1+ subset, and by the low level of c-kit expression. The CD34+/Thy1+ subset appears phenotypically and functionally heterogeneous, a finding consistent with its high representation, compared to phenotypes such as CD34+/CD38- . Therefore, while at least some of CD34+/Thy1+ cells may be infectable by retroviral vectors, as shown by the presence of a transcript for the receptor for murine amphotropic retroviruses, the use of this selection strategy to specifically target human stem cells appears questionable.

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