Part 2: Patients will be hospitalized at the NIH Clinical Center for 5 days, undergoing the same procedures as in Part 1, except they will receive G-CSF for 5 days instead of 2. They will also undergo the same procedures detailed for the 2-week and 1-month follow-up visits. Then, at 3 months they will again return to the clinic for a physical examination, routine blood tests, and a treadmill exercise test. This concludes formal participation in the study. However, patients are requested to return to the outpatient clinic once a year for follow-up.