Monoclonal B-cell lymphocytosis (MBL) is defined by the presence of a monoclonal B-cell population in the peripheral blood (PB) at a concentration of <5 × 109/l and no signs or symptoms of a lymphoproliferative disorder. In around 75% of cases, the immunophenotype of the clonal B-cell expansions is superimposable to that of chronic lymphocytic leukemia (CLL), thus defined “CLL-like”. Other cases may coexpress CD19, CD5, bright CD20, and lack CD23 (“atypical CLL”), while others are CD5-negative (“non-CLL”).
