Flow cytometry of day 19 fetal livers showing separation of B-lineage subsets in Eμ-ret (Tg+) and transgene-negative (Tg−) mice. In the upper diagrams, B-lineage cells (CD45R⁺) are resolved from total liver cells into less differentiated (CD43⁺) and more differentiated subsets (CD43⁻). The percentages of CD45R⁺CD43⁻ (left upper quadrant) and CD45R⁺CD43⁺ (right upper quadrant) cells among total liver cells are shown in the corresponding quadrant. In the lower diagrams, CD45R⁺CD43⁺ cells are further resolved into BP-1⁻ early pro-B (bottom region), CD24⁺BP-1⁺ late pro-B (upper left region), and CD24⁺⁺ BP-1⁺ early pre-B cells (upper right region) and their percentages are shown in their corresponding region. An arrow points to a population of late pro-B cells that expresses high levels of BP-1, as previously observed in the bone marrow of 3-to 5-week-old Eμ-ret mice.