Screening tests include a complete physical examination and medical history, during which patients will provide a list of current medications or supplements they are taking. There will be a collection of about 4 tablespoons of blood for analysis of blood counts as well as liver and kidney function. Patients may also undergo a magnetic resonance imaging (MRI) scan of their brain, but the procedure is optional. During the MRI, they will lie on a table that will slide into the enclosed tunnel of the scanner. The MRI takes about 20 to 30 minutes, and patients will be asked to lie as still as possible. There will also be a bone marrow biopsy, if patients have not had one done within 4 weeks of the start of this study. Eligible patients will take a 10 mg dose of metoclopramide by mouth, three times a day, for 20 weeks. They will be given a 4-week supply, which will be renewed monthly at each treatment visit. It is essential that patients be seen at NIH during the first, third, and fifth months of the study. Visits made in the meantime, at the second and fourth months, may be done at the office of their doctors who have referred them for the study, or at NIH. During the treatment visits, patients will be asked to update their medical history, health conditions, and use of medications or herbal supplements. There will also be a collection of about 1 tablespoon of blood for laboratory tests. Patients will be asked to make a similar follow-up visit 1 month after they stop taking metoclopramide, so that the response to treatment can be evaluated.