The effect of ATRA on cell growth and colony formation from lin⁻c-kit⁺Sca-1⁻hematopoietic progenitor cells. One hundred FACS-enriched hematopoietic progenitors (lin⁻c-kit⁺Sca-1⁻) were added to wells containing media and cytokines (SCF, IL-6, IL-11, and Flt-3-ligand) and cultured without (▪) or with (•) 1 μmol/L ATRA. Cells were removed from liquid suspension at periodic intervals, washed, counted, and replated in semisolid medium without ATRA. The cell counts (A and B) and total colony outputs (C and D) are given as the total number per 100 starting hematopoietic progenitors. Results are shown from two separate experiments and are expressed as the mean ± SEM from 5 separate wells for each time point.