Residues throughout the E2A-PCET motif are critical for oncogenesis. (A) Proliferation of myeloid progenitors associated with retroviral-mediated expression of wild-type E2A-PBX1b and engineered E2A-PBV1b variants. For each construct, 3 × 10⁵ bone marrow cells were cultured in GM-CSF immediately after retroviral infection and counted for 35 days. (B) Western blot of lysates from NIH 3T3 fibroblasts infected with retroviruses forcing expression of the indicated E2A-PBX1b constructs, confirming expression of all the different mutants of E2A-PBX1b. The values presented below the blot represent expression levels of the various E2A-PBX1 mutant constructs, relative to wild-type E2A-PBX1, after normalization according to GFP expression.