Lyn^-/- mice have elevated numbers of CFU-S₁₂ in the spleen but are more severely affected by the cytotoxic drug 5-FU. (A) Numbers of primitive hematopoietic progenitors in the spleen and BM of Lyn^+/+ and Lyn^-/- mice capable of forming macroscopic colonies in the spleens of irradiateSGGd recipient mice 12 days after injection of BM or spleen cell populations (^*P < .01, Student t test). Total cellularity of the (B) BM, (C) spleen, (D) peripheral blood, and (E) hematocrits of Lyn^+/+ and Lyn^-/- was determined at 2-day intervals following intravenous injection of the cytotoxic drug 5-FU. Results for spleen and blood are shown as percent of untreated, since young C57BL/6 background Lyn^-/- mice have diminished splenic and white blood cell (WBC) counts compared with control mice. (F) The total cell number of CD71⁺Ter119⁺ double-positive cells in the spleen of 5-FU-treated Lyn^+/+ and Lyn^-/- mice determined by FACS analysis at 2-day intervals over 16 days. In experiments depicted in panels B-F, 5 age- and sex-matched mice per genotype were analyzed at each time point. Data presented are the mean (± SEM). (G) Numbers of BM CFU-e's and BFU-e's were assessed 3 days after challenge with 5-FU. Data represent the mean (± SD) of 3 to 4 mice/genotype. (H) Percentage BrdU^pos erythroid (CD71⁺) and myeloid (Mac-1⁺) cells in BM 2.5 hours after injection of BrdU (40 mg/kg). Data presented are the mean (± SD) of 3 mice per genotype with similar results observed in 2 independent experiments.