Inhibition of p38 MAPK enhances long-term repopulating ability of WT and Fancc^−/− HSCs. WT and Fancc^−/− low-density MNCs were cultured with either a vehicle control or SB203580 for 4 days. Cultured cells were enumerated and used as donor cells in competitive repopulation studies. Two separate transplantation experiments were conducted with 6 or 7 recipients transplanted for each of 4 experimental groups: WT plus vehicle control, WT plus SB203580, Fancc^−/− plus vehicle control, and Fancc^−/− plus SB203580. Peripheral blood donor chimerism from individual transplant recipients from both experiments were combined (a total of 53 transplanted mice). Data shown illustrate mean donor chimerism of 12 to 14 recipients from each of the 4 experimental groups at 3, 6, and 9 months after transplantation. *P < .05, comparing SB203580 to vehicle control within each genotype.