Reason for this study: To understand how exercise prevents cancer. What you will do: Fitness assessments, questionnaires, supervised exercise, blood and saliva collection. Compensation: You can earn up to $300. Who can be in the study: You must be a female between the ages of 30-45 and plan to live in Denver for the next 10 months. You should not be a regular exerciser but you should be willing to exercise four times per week for 16 weeks.