CLL almost uniformly evolves from the precursor condition monoclonal B-lymphocytosis (MBL) characterized by the co-expression of CLL-like surface markers (CD19, CD5, and CD23, weak expression of CD20 and CD79b) with less than 5 x 109/L B-cells in the circulation. The gene discussed is CD5; the disease is B-cell chronic lymphocytic leukemia.