Effect of a c-Myb-deficient genetic background on hematopoietic transformation. Mice heterozygous for a hypomorphic allele of c-Myb (Myb^kd) were intercrossed and the genotype of the offspring was determined by PCR. Myb^kd is a floxed-neo cassette inserted into intron 6 of the c-Myb gene.³⁷  This modification leads to a reduction of c-Myb protein expression to approximately 5% to 10% of the wild-type level. Hematopoietic precursors from 5-FU-treated bone marrow harvested from 8- to 10-week-old mice of the indicated phenotype were transduced with MLL-ENL- or E2A-HLF-expressing retroviruses. Cells were subject to standard serial replating in methocel. A typical example of third-round colonies is shown. In total the experiment was performed 3 times.