Because our findings indicate that only IgM BCR amounts and aggregation into high-density regions — parameters linked with BCR signaling — associate with in vivo clonal BRs (Figure 1 and Supplemental Figure 3F) and cellular changes associated with BCR signaling (Figures 2 and 3 and Supplemental Figures 4 and 5), we investigated whether autonomous BCR signaling, which is expected to be an ongoing event in CLL clones, differed for the IgM and IgD isotypes. This evidence concerns the gene BCR and B-cell chronic lymphocytic leukemia.