A) Diffuse large B-cell lymphoma of the stomach; CD20 immunoreactivity evidenced in 100% of neoplastic cells by L26 clone monoclonal antibody. B) Histopathological picture at relapse, resulting in CD20 (L26)-negative phenotype; CD79a immunoreactivity (insert) confirms the B-cell immunophenotype of neoplastic cells. C) CD20 immunoreactivity (arrows) in near 10% of neoplastic cells at the second relapse, detected by using L26 clone monoclonal antibody. The use of a second antibody against the C-terminal intracellular domain of the transmembrane CD20 molecule (7D1 clone, Novocastra, Newcastle, UK) displayed overlapping results when compared with L26 antibody (data not shown). All immunohistochemistry techniques were performed using Novolink Polymer Detection Kit (Novocastra, Newcastle, UK) according to manufacturers’ instructions.