Individuals interested in becoming prospective bone marrow donors and joining the Registry of the National Marrow Donor Program (NMDP) should do so by calling 301-496-0572, the phone number of the NIH Marrow Donor Center. An appointment will be made to have a health history taken and to have a buccal swab collected for tissue (HLA) typing. The current protocol only involves donors already registered with the NMDP, who have been identified as "perfect matches" for an unrelated patient requiring a bone marrow transplant, and who require expedited medical evaluation prior to marrow collection.