The antibody chosen for Bexxar and Zevalin specifically attaches to CD20, a protein on the surface of some normal white blood cells and often on white blood cells that transform into the malignant cells of non-Hodgkin's lymphoma. The nonradioactive form of the drug, Rituxan, has proved to be a powerful tool in the treatment of this cancer. But some tumors (even with CD20 protein present on their cell surfaces) don't respond well enough to Rituxan, and some tumors initially respond and then become resistant to treatment. Even in these cases, Bexxar and Zevalin seem to work pretty well.