The reduction of SDF-1 responsiveness during B lymphopoiesis is disproportionate to the decrease in CXCR4 expression. The terms “CXCR4 positive cells” and “anti-CXCR4 (12G5) antibody binding sites per cell” are defined in Materials and Methods. Presence of calcium mobilization after stimulation describes the percentage of cells responding with calcium mobilization after SDF-1 stimulation. Migration of cells toward SDF-1 describes the ratio between the number of cells that migrated in the presence of chemokine and the number of cells that migrated in the presence of media alone. CXCR4 expression and function data of late lineage (immature and mature) B cells are normalized to the data of early (pro-B and pre-B) lineage B cells (defined as equal to 1). The data represent the mean of experiments shown in Figs 2 and 4 and Table 2.