Once you complete the main study, you will be offered the opportunity to participate in a substudy, where you will repeat the same visits, except that if you chose to have all your samples taken in the clinic for the main study, you will take certain samples at home for the substudy. If you chose to take certain samples at home for the main study, you will have all the samples taken at the clinic for the substudy. Also, you will not need to do another screening visit if you start the substudy within 9 weeks of the end of the main study.