Establishment of MlFosER cell lines. M1 cell lines that conditionally express c-Fos (MlFosER) levels comparable to myeloblasts cultured with LUCM for 3 days have been established by infection with a retroviral vector containing the chimeric foser transgene, under control of the MSCV promoter. The murine c-fos cDNA was ligated to the hormone-binding domain of the human estrogen receptor (ER), encoding for a chimeric FosER protein whose activity is dependent on the presence of β-estradiol. RNA expression was analyzed by hybridization of a c-fos probe to Northern blots, using total RNA (10 μg/lane).