Background: \- Fatigue is a very common early and late side effect of cancer treatment, including radiation therapy. The cause of fatigue is poorly understood, making it hard to diagnose and treat. More research is necessary to understand why patients receiving cancer treatment experience fatigue. Changes in mitochondria, parts of body cells that help provide energy to the cell, may contribute to fatigue. Researchers are interested in looking at blood chemicals and mitochondrial genes of cancer patients to study those associated with fatigue. Objectives: \- To study the relationship between fatigue and the effects of cancer treatment. Eligibility: * Men at least 18 years of age who have been diagnosed with localized prostate cancer and are scheduled to receive external beam radiation therapy. * Participants on study 09-NR-0088, Molecular-Genetic Correlates of Fatigue in Cancer Patients Receiving External Beam Radiation Therapy, are also eligible. Design: * This study requires three outpatient visits to the NIH Clinical Center. * Participants will be seen before they start radiation treatment, at the middle of treatment, and at the end of treatment. Each visit will take less than 30 minutes to complete. * Participants will complete questionnaires that ask about fatigue and depression. * Participants will provide blood samples for research testing and potential HIV testing. * No treatment will be provided as part of this protocol.
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