The differential diagnosis for HCL regardless of the site of involvement includes all forms of mature B-cell lymphomas including marginal zone lymphoma (MZL), splenic lymphoma with villous lymphocytes (SLVL), B-cell prolymphocytic leukemia (B-PLL), and very rarely mantle cell lymphoma (MCL) or atypical CD5 negative chronic lymphocytic leukemia/lymphoma (CLL/SLL) and lymphoplasmacytic lymphoma. The gene discussed is CD5; the disease is hairy cell leukemia.