One of our additional studies is a clinical trial of the drug rapamycin, also called sirolimus, a prescription medication that is used in humans to treat cancer and to prevent organ rejection in transplant patients. When used at very low doses in laboratory mice, rapamycin seems to positively affect the way their bodies age. We are interested in discovering whether these benefits could be seen in dogs as well and whether there are unwanted side effects. Most dogs enrolled in the Dog Aging Project will not receive this drug, however, a small group of dogs, who meet specific age, weight, and location criteria, will be invited to participate. Each individual owner can decide whether or not to participate at that time. Participation in this portion of the study is completely voluntary. We do not recommend the use of this drug outside of our controlled clinical trial.