CD20 protein–negative phenotypic changes in CD20-positive B-cell lymphoma patients after treatment with rituximab-containing chemotherapy. Tissue samples (LN, lymph nodes; CS, bone marrow clot section) obtained from UPNs 1 (A), 4 (B), and 3 (C) in Table 2 were analyzed by IHC using anti-CD20, anti-CD79a, and anti-CD10 antibodies. Anti-CD79a antibody was used for detection of B cells. Note that CD20 was positive at the time of initial diagnosis in these patients, and that the CD20-negative phenotypic change was observed during the relapse/progression period. Original magnifications, ×400 (A) and ×200 (B,C) (Olympus BX51TF microscope, Olympus, Tokyo, Japan, and Nikon DS-Fi1 camera, Nikon, Tokyo, Japan). HE indicates hematoxylin and eosin staining; and RD/PD, relapse/progression of disease.