TGF-β1 played a major role in preventing HSC death and maintained HSCs in quiescence. LSK BM cells from 8- to 10-day-old TGF-β1^+/+ and TGF-β1^−/− mice were single cell sorted into wells of Terasaki plates and incubated in serum-free medium supplemented with cytokines. At 5 days of culture, we determined (A) cell viability as a percentage of wells containing no cell over the total number of wells plated, (B) cell quiescence as the percentage of wells containing 1 living cell over the total number of wells containing viable cells, and (C) proliferation as the percentage of wells containing 2 or more cells over the total wells containing viable cells. Wells were examined under an inverted microscope. (D) Sorted LSK BM cells (1 × 10⁵) from 8- to 10-day-old TGF-β1^+/+ and TGF-β1^−/− mice were stained with CFSE. CFSE⁺ LSK cells were sorted into a 6-well plate and incubated in serum-free medium supplemented with cytokines. Number of cell divisions was determined by the percentage of CFSE⁺ cells by flow cytometry during 96 hours. Graphs show the mean ± SD of 3 experiments (6 mice per experiment); *P < .05.