Classic hairy cell leukemia (HCL) is a rare B-cell malignancy expressing CD11c, CD22, CD25, CD103, CD123, annexin A1 (Anxa1), and BRAF V600E mutation.1,2 Purine analogs cladribine and pentostatin achieve complete remissions (CR), but relapse occurs at a median 4.5-16 years depending on age and whether follow-up included bone marrow or just blood counts.3,4 Retreatment is effective but with cumulative toxicities and relapses of decreasing intervals. The gene discussed is CD22; the disease is hairy cell leukemia.