Classically, diagnosis of HCL was confirmed by tartrate-resistant acid phosphatase activity, although the standard practice today is immunophenotyping by flow cytometry where HCL is characterized by the expression of B-cell antigens CD19, CD20, and CD22 in addition to coexpression of the surface antigens CD11c, CD25, and CD103. This evidence concerns the gene CD22 and hairy cell leukemia.