For that purpose, we treated animals of all three genotypes at the time of overt CLL manifestation (defined as >20% of all CD45+/SSlow lymphocytes displaying the CD5+/CD19+ immunophenotype of CLL cells, in the peripheral blood) with cyclophosphamide (200 mg/kg, i.p., on days 1, 8, 15, and 22). This evidence concerns the gene CD5 and B-cell chronic lymphocytic leukemia.