Flow cytometric studies were employed to determine within B-cell populations the percentage of CXCR4 positive cells, the mean number of anti-CXCR4 ABS and migration response to SDF-1. Analyses of early lineage B cells were performed on a cell population that included both pro-B (CD19⁺, κ⁻/λ⁻, CD34⁺) and pre-B (CD19⁺, κ⁻/λ⁻, CD34⁻) cells, whereas analyses of late lineage B cells were done on cell population that included both immature (CD19⁺, κ⁺/λ⁺, IgD⁻) and mature (CD19⁺, κ⁺/λ⁺, IgD⁺) B cells (see Fig 1A). The values representing the percentage of cells responding with calcium flux after SDF-1 stimulation were derived from experiments with sorted cells (Fig 3). The value given for early lineage B cells was obtained by combining the data from 4 experiments, each of pro-B and pre-B cells. The value given for late lineage B cells reflects the combined results of 4 experiments including sorted mature B cells only.