The effect of ATRA on cell growth and colony formation from lin⁻c-kit⁺Sca-1⁺hematopoietic precursor cells. Fifty FACS-enriched hematopoietic precursors (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 colony outputs of CFU-GM (C and D) and HPP-mix (E and F) are given as the total number per 50 starting hematopoietic precursors. Results are shown from two separate experiments and are expressed as the mean ± SEM from 5 separate wells for each time point. Two additional experiments were evaluated periodically over 21 days, each with similar results, with ATRA-treated cells exhibiting an initial slow growth followed by an accelerated cell growth associated with enhanced CFU-GM and HPP-mix production.