Cellular filiation in the Ubow mouse. (A) Schematic describing that when Ubow cells express Cre, they simultaneously lose dTomato and acquire CFP or YFP expression. Applied to a precursor/product relationship and in combination with the proper Cre reporter strain, this system unravels the transitional stage between progenitors and their differentiated cells. (B) Non-chimeric Ubow^+/−-CreERT2 mice were injected with a single dose of tamoxifen. Mice were bled every day and the relative proportion of CFP-, YFP-, and dTomato-expressing B220⁺ B cells were assessed by flow cytometry (same results were observed in CD3⁺ T cells). Data are representative of 3 different mice from 3 different experiments. (C) Schematic showing the expected phenotypes and colors of MRCs (RANK-L^hi CD21^neg) and immature (RANK-L^med CD21^med) and mature FDCs (RANK-L^neg CD21^hi) in CD21Cre Ubow mice according to the model in which MRCs give rise to FDCs.