Transfections For the HPC7 line, 5 x 106 cells were electroporated with up to 6µg of DNA using the Amaxa 4D-nucleofector with the solution L and program X-001, or the Amaxa 4D-nucleofector with the SF cell line solution and the DS-120 program (Biosystems), respectively. C/EBPa and MYB expressing plasmids or corresponding shRNA vectors (Origene) were co-transfected with a GFP-encoding vector (Amaxa) and GFP-expressing cells were sorted on a MoFlo XDP cell sorter (Beckman Coulter). Transfections of primary cells were performed on a population enriched in bone marrow KIT+ cells. Following the enrichment step using a MoFlo XDP cell sorter (Beckman Coulter), the cells were cultured 2 hours prior to transfection in IMDM medium supplemented with 5%FBS, 50u/ml penicillin, 50µg/ml streptomycin, 0.1mM non-essential amino acids, 2mM L-glutamine, 1mM sodium pyruvate, 50µM 2-mercaptoethanol, 10ng/ml GM-CSF, 25ng/ml SCF, 25ng/ml IL11, 10ng/ml IL3, 25ng/ml TPO and 4u/ml EPO. We used the Amaxa 4D-nucleofector with the solution P3 Primary Cells and program DS-138 to electroporate sets of 1 to 2 x 106 cells with 0.5µg of the Amaxa pmax GFP plasmid and 600nM siRNA in total. For the single knockdowns 300ng of Myb (s70212, Ambion—Lifetechnologies) or Cebpa siRNA (s63855, Ambion- Lifetechnologies) were combined with 300nM of siRNA control (4390843 Silencer Select Negative Control #1—Lifetechnologies) to maintain comparability with the double knockdown results. Transfected cells were cultured for 20 hours in the aforementioned IMDM-based medium, analysed by flow cytometry and GFP expressing KSL cells were sorted for quantitative PCR analysis.