BCR and B-cell chronic lymphocytic leukemia: Although cumulatively, BCR stereotyped subsets are common, encompassing up to 41% of all CLL, the prevalence of each individual subset is low: the largest subset, subset #2 (defined as patients with a BCR comprised of IGHV3-21, IGLV3-21, with a short, stereotypic heavy-chain complementarity-determining region of 9 amino acids), represents around 2.5% of all patients (128).