Progression of Lin⁻/Sca⁺ cells synchronized at G₁ /S boundary by aphidicolin treatment through S phase. Freshly isolated Lin⁻/Sca⁺ cells were incubated in medium containing 7 cytokines. From 8 to 32 hours thereafter, cells were treated with aphidicolin as indicated by the arrows. Cells were then washed once with cytokine-free medium and incubated with medium containing either 7 cytokines or SCF alone. At indicated intervals, duplicate aliquots of cells from each group were analyzed for ³H-thymidine incorporation followed by autoradiography to assess the percentage of cells with labeled nuclei as described in the Materials and Methods.