CD19 and B-cell chronic lymphocytic leukemia: We analyzed 14-month-old mice and their control littermates (negative for mb1-Cre) for evidences of a CLL and found that the number of B cells in spleen and blood (Fig. S6c), the presence of CD19 + CD5+ cells in blood (Fig. S6d), and the distribution of B cells among follicular, marginal zone and CD21−CD23− B cells (Fig. S6e) were within normal limits.