Fludarabine is a purine nucleoside analog of adenosine that is clinically used to treat leukemias such as chronic lymphocytic leukemia (CLL) and also to treat graft-versus-host disease (GVHD) in transplant patients. Fludarabine exhibits anticancer chemotherapeutic and immunosuppressive activities. Incorporation of fludarabine into DNA chains inhibits ribonucleotide reductase, DNA ligase, and DNA primase, inducing chain termination and preventing DNA synthesis. In vitro, fludarabine inhibits TNF-α-stimulated degradation of IκB kinase, preventing activation of NF-κB. This compound also induces cell cycle arrest and apoptosis in alloreactive bone marrow stromal cells. Like other adenosine analogs, fludarabine also acts as an antagonist at the A1 adenosine receptor.