BM MSCs from infants with MLL-AF4+ pro-B ALL harbor and express the MLL-AF4 fusion gene. (a) Fluorescence in situ hybridization performed in patient-derived MSCs (top row) and leukemic blasts (bottom row) (n=38). Leukemia-specific fusion genes were always observed in the leukemic population. Using a split-apart probe, MLL rearrangements are identified by the presence of one red signal, one green signal and one yellow signal (germline). Using locus-specific probes, the fusions TEL-AML1, AML1-ETO and BCR–ABL are determined by the presence of yellow fusion signals (and the derivative chromosome) whereas cells without the translocation have two green (either BCR, TEL or ETO) and two red signals (either ABL or AML1). The white arrows depict the rearranged allele. Bar, 100 μm. (b) Representative quantitative reverse transcriptase PCR experiments performed in duplicate from two patients showing MLL-AF4 transcript expression in MSCs from infants with MLL-AF4+ pro-B ALL.