- Chronic lymphocytic leukemia (CLL), small lymphocytic lymphoma (SLL), and mantle cell lymphoma (MCL) are types of cancers in which there are too many abnormal lymphocytes (a type of white blood cell). Monoclonal B-cell lymphocytosis (MBL) is a condition in which the individual has a larger than normal number of lymphocytes. Individuals with CLL, SLL, MBL, and MCL may survive for many years without the need for treatment, but there is an apparent correlation between cell birth rates and disease activity. By studying the birth and death rates of lymphocytes, researchers hope to identify individuals who are at risk for worsening disease.