The purpose of the study is to determine how effective preemptive tenofovir therapy is in
preventing the re-activation of Hepatitis B infection, in patients who are receiving
rituximab-based chemotherapy for Non-Hodgkin's Lymphoma or CLL/SLL. The rate of re-activation
will be compared between patients who receive preemptive tenofovir and patients who receive
tenofovir as needed.