Analysis of hematopoiesis using the spleen colony formation assay. (A) Representative spleens of irradiated mice that received 10⁵ bone marrow cells 8 or 12 days before analysis. (B) Graph showing the average number of colonies per spleen at day 8 or day 12 after transplantation. The difference between genotypes in colony number at day 8 was statistically significant by Student t test (P < .01).