Patients can spend up to six years waiting for a kidney transplant. Even when they do receive a transplant, up to 20 percent of patients will experience rejection. Transplant rejection occurs when a recipient’s immune cells recognize the newly received kidney as a foreign organ and refuse to accept the donor’s antigens. Current methods for testing for kidney rejection include invasive biopsy procedures, causing patients to stay in the hospital for multiple days. A study by investigators from Brigham and Women’s Hospital and Exosome Diagnostics proposes a new, noninvasive way to test for transplant rejection using exosomes—tiny vesicles containing mRNA—from urine samples. Their findings are published in the Journal of the American Society of Nephrology.