HSC-SCL-Cre-ER^T–mediated recombination in CFU-S₁₂ of adult bone marrow. Lethally irradiated recipients were transplanted with 7.5 × 10⁴ bone marrow cells of tamoxifen-treated HSC-SCL-Cre-ER^T;R26R-EYFP mice (n = 4, 2 mg tamoxifen per day for 14 days, transplanted 2 weeks after last injection). The recipients were killed 12 days later, and the spleens were harvested for analysis. (A) EYFP-positive CFU-S₁₂ colonies were visualized with a fluorescent dissection microscope (left panel, bright field; right panel, eGFP filter). (B) Single CFU-S₁₂ colonies were microdissected, stained for Ter119, and analyzed by flow cytometry. Representative plots of an EYFP-negative (left panel) and an EYFP-positive colony (right panel) are shown. Percentages refer to the proportion of EYFP-positive cells within single microdissected CFU-S₁₂ colonies.