Composition of the BM precursor B-cell compartment of 2 patients with low transitional and memory B cells (B-cell pattern 1). Using flow cytometric immunophenotyping, 4 major precursor B-cell subsets can be identified (pro-B, pre-B-I, pre–B-II, and immature B). In healthy donors, the immature B-cell fraction comprises 25% of the total precursor B-cell compartment. The proportion of immature B cells in CVID patients with pattern 1 was decreased compared with controls.