The immunophenotype of the B cell clones in approximately 75% of all MBL cases is similar to that observed in chronic lymphocytic leukemia (CLL): CD5+, CD19+, CD20dim, CD23+ and low levels (dim) of surface immunoglobulin (sIgdim) that in most cases is IgM with or without IgD. This evidence concerns the gene FCER2 and B-cell chronic lymphocytic leukemia.