Chronic lymphocytic leukemia (CLL) is the most common leukemia in the Western world and accounts for approximately 11% of all hematologic malignancies.(1) CLL affects approximately 3–5 out of every 100,000 individuals in the U.S. and is presently considered a treatable but incurable illness with conventional therapies. Remarkably, while conducting a public health study in 1991, the U.S. Center for Disease Control (CDC) identified a clonal population of B-cells with CLL phenotype in 9/1499 (0.6%) individuals in the general population age 45 or older.(2, 3) This prevalence of roughly 1 out of every 170 individuals was several hundred times more common than that of CLL. Subsequent studies found that clonal B-cell populations with the phenotype of other low grade, non-Hodgkin lymphoma (NHL) subtypes (e.g. marginal zone) were also prevalent in the general population.(4, 5) To standardize characterization and promote future scientific investigation, an international panel of experts designated the presence of such clones “monoclonal B-cell lymphocytosis” (MBL) and proposed formal criteria for the diagnosis of MBL in 2005.(6) These criteria also sub-classified individuals with MBL as having i) CLL-like phenotype ii) atypical CLL phenotype, or iii) non-CLL phenotype based on surface protein expression (Table 1). (6)
