Reconstitution of Pax5 expression rescues early B-cell development in p100^−/− bone marrow. Bone marrow from 3-week-old mice was analyzed by flow cytometry for CD19 and B220 (c-Kit⁺CD43⁺ gate). Ik^neo/+, transgenic mice expressing an inactive Pax5 transgene under the control of the Ikaros locus; Ik^neo/+p100^−/−, Ik^neo/+ mice on a p100^−/− background; Ik^Pax5/+p100^−/−, p100^−/− mice in which the inactive Ik^neo allele has been converted into the active Ik^Pax5 transgene upon Cre-mediated recombination using a Vav-Cre mouse line. Numbers represent mean values (± SD) from 3 mice per genotype.