- CD19 expression is required and must be detected on greater than 50% of the malignant cells by immunohistochemistry or >= 90% by flow cytometry; the choice of whether to use flow cytometry or immunohistochemistry will be determined by what is the most easily available tissue sample in each subject; in general, immunohistochemistry will be used for lymph node biopsies, flow cytometry will be used for peripheral blood and bone marrow samples