5. Relapsed and/or refractory disease to a purine nucleoside analogue (pentostatin, fludarabine, or cladribine). Relapse is defined as a patient who has previously achieved the clinicopathologic criteria for a CR or PR, but after a period of ≥ 6 months demonstrates evidence of disease progression. Refractory is defined as a patient progressing on therapy or who cannot maintain at least a PR for ≥ 6 months (Appendix IV). The patient may have had therapy subsequent to receiving a purine nucleoside analogue, but must also have relapsed or been refractory to this most recent therapy (Appendix IV).