A drug used to treat chronic lymphocytic leukemia (CLL) that has not gotten better with other anticancer drugs. It is also used with chlorambucil in patients who have not already been treated and cannot receive certain anticancer drugs. It is also being studied in the treatment of other types of cancer. Arzerra binds to a protein called CD20, which is found on B cells (a type of white blood cell) and some types of leukemia and lymphoma cells. This may help the immune system kill cancer cells. Arzerra is a type of monoclonal antibody. Also called HuMax-CD20 and ofatumumab.