Hairy cell Leukemia (HCL) is a chronic lymphoproliferative disorder that is defined, according to the WHO classification, as a mature (peripheral) B-cell neoplasm. HCL accounts for between 2-3% of all leukemia cases, with about 600 new cases diagnosed in the U.S. each year. With regard to genetic abnormalities of potential pathogenetic importance in HCL, only a few have been described. Mutations of p53 and Bcl-6 have been reported in about one-third of cases, but their functional significance is unclear. Over-expression of cyclin D1, an important cell cycle regulator, has been reported in cases of HCL as well as mantle cell lymphoma (MCL) and in a few cases of chronic lymphocytic leukemia among lymphoid malignancies.