CD5-negative phenotype of monoclonal B-lymphocytosis of undetermined significance (MLUS).

Monoclonal B lymphocytosis of undetermined significance (MLUS) is a benign condition of clonal lymphocytosis. Because of its clonal small lymphocytes and indolent clinical course, MLUS resembles and may be indistinguishable from an early stage of chronic lymphocytic leukemia (CLL). In this study we characterized the immunophenotype of MLUS. The clonal B lymphocytes from three patients of typical MLUS were shown to be CD5(-) and CD23(-) phenotype. Their immunophenotypic features remained unchanged during the clinical course. The results indicate a distinct immunophenotype of MLUS that can be useful for its diagnosis and differentiation from CLL.