Expression of differentiation markers on BM-derived B-lineage cells from a healthy child, patient 2, and a patient with a mutation in Btk (W588X). Cells were stained with APC CD19, PE CD37, and APC-Cy7a CD20, then permeabilized and stained with FITC TdT. The top panel shows all cells that were positive for either CD19 or TdT with gates drawn to identify TdT⁺/CD19⁻, TdT⁺/CD19⁺, and TdT⁻/CD19⁺ populations. The expression of CD20 and CD37 on each of these populations is shown in the bottom panel.