The effect of ATRA on the growth of single lin⁻c-kit⁺Sca-1⁺hematopoietic precursor cells. Single 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. After sorting, each well was individually viewed for the presence of a single cell, and wells in which no cell or more than one cell were initially observed were not included in the experiment. The number of cells in each well were counted at daily intervals after culture initiation. The cumulative percentage of wells containing at least one cell division (A) and the percentage of wells with 1 to 16, 17 to 50, and greater than 50 cells per well at daily intervals (B) are shown. In (B), (▪) indicates no ATRA and () indicates ATRA. Results are shown from one experiment but are indicative of three separate experiments, each initiated with 120 single cells per treatment.