Examples of detection of rituximab and CD20 in 4 different patients with CLL being treated with rituximab. Varying degrees of masking of CD20 expression (A) and detection of rituximab (B) on CLL cells are shown. In patients 1 and 2 almost all the B cells are positive for rituximab, whereas no rituximab is detected on the surface of cells from patients 3 and 4. Some of the B cells were CD20⁺ despite the negativity for rituximab. All B cells were negative for both CD20 and rituximab, which suggests that CD20 is no longer expressed on the surface of cells rather than masking by the rituximab. CD20 expression was detected in more than 90% of CD19⁺ cells in all patients before therapy. The percentage of cells in each quadrant is shown in each plot.