Aldh1a1 is non–cell-autonomously required to reduce the toxicity of cyclophosphamide. (A) Aldh1a1^−/− and littermate control mice were treated with 250 μg G-CSF/kg body mass/day for 4 days to induce HSC mobilization. Aldh1a1 deficiency did not significantly affect bone marrow or spleen cellularity or HSC content (n = 2-5 mice per treatment in 2 independent experiments). (B) Littermates were treated with 200 mg/kg cyclophosphamide (CY) followed by 4 days of 250 μg/kg G-CSF. Aldh1a1^−/− mice had significantly (*P < .05) fewer HSCs in their spleen compared with littermate controls but did not exhibit a significant difference in overall bone marrow or spleen cellularity (n = 3-6 mice per treatment in 2 independent experiments). (C) Donor-cell chimerism in wild-type mice reconstituted with Aldh1a1^−/− (red lines) or littermate control (black lines) bone marrow cells that were treated weekly with 200 mg/kg cyclophosphamide. Because cyclophosphamide did not generally reduce the levels of Aldh1a1^−/− donor cells compared with control donor cells these results indicate that the toxic effect of cyclophosphamide on HSCs is primarily non–cell-autonomously attenuated by Aldh1a1. (D) Survival of Aldh1a1^−/− and littermate control mice treated weekly with 150 mg/kg 5-fluorouracil. Data represent the combined results from 2 independent experiments (n = 20-22 mice per treatment). Error bars represent SD.