It is possible that many of the subjects with detectable clonal mosaicism in our study have monoclonal B-cell lymphocytosis (MBL), an asymptomatic condition with an estimated prevalence of 3–5% in the elderly. MBL is characterized by a clonal population of B lymphocytes with an immunophenotype similar to CLL or other B-cell malignancy41. Most, if not all, cases of CLL are preceded by MBL, but most cases of MBL do not progress to malignancy42,43. However, 85% of MBL detected in population screening studies have a B-cell count below 500/μl43, which is less than 10% of the normal white blood cell count. Since 10% is near the lower limit of detection for chromosomal mosaicism using our methods, the two types of clones may not be closely related. Nevertheless, further work on the relationship between B cell immunophenotypes and mosaic anomalies is warranted.
