Flow cytometric analysis of peripheral blood lymphocytes from a healthy control, patient 2, and a patient with a mutation in Btk (8-bp deletion in exon 8). Cells were stained with PE-labeled anti-CD19 and separate aliquots were stained with FITC-labeled polyclonal anti-IgM, a combination of anti-κ and anti-lambda, monoclonal anti-CD22, CD38, or CD21. For the control samples, approximately 20 000 events were analyzed; for the patient samples, between 100 000 and 500 000 events were analyzed.