Experimental evidence, combined with clinical observations such as the fact that the transformation of CLL to large cell lymphoma (Richter’s Syndrome) and the progression toward a more malignant phase of the disease characterized by an increased number of prolymphocytes is often associated with immunophenotypic drift where CD5 is lost, supports the notion that CD5 is important in CLL development and proliferation. The gene discussed is CD5; the disease is anaplastic large cell lymphoma.