Time-course of proliferation of infected fetal liver cells in liquid culture. Cells were infected with RufNeo retroviral constructs containing the indicated hβc subunits as described in the Materials and Methods. Cells were then washed and 2 × 10⁵ cells were cultured with (A) and without (B) 500 U/mL mIL-3 and mGM-CSF and 2 U/mL hEpo. Cell counts were performed at weekly intervals. (□) hβc; (▴) FIΔ; (▾) V449E; (⋄) I374N.