Assessment of hairy cells in a patient with HCL by flow cytometry performed on bone marrow aspirate specimen. CD19⁺ lymphocytes are shown. (A) Day 0, hairy cells are 10.7% of total cells. (B) Day 30 (after 2CDA), hairy cells are 11.0% of total cells. (C) Day 90 (after rituximab), hairy cells are undetectable, with fewer than 1/200 000 cells CD19⁺ with CD103 expression significantly above background.