CD5 and B-cell chronic lymphocytic leukemia: As discovery of B-1 cells began with the identification of CD5 (Ly-1) on the surface of cancerous B cells in patients with B cell chronic lymphocytic leukemia (19, 20), the presence of CD5 on murine B cells that were long-lived, self-renewing, producers of polyreactive IgM antibodies has been regarded by many as synonymous with murine B-1 cells.