TRIAD is designed to study the effects of rapamycin on aging dogs. We know that size affects the rate of aging in dogs. Including both large and small dogs in this study would introduce another variable (size-specific aging rate) and make it harder for us to detect any effects related to rapamycin. In addition, size affects metabolic rate, meaning that large and small dogs will metabolize drugs like rapamycin differently. For this clinical trial, we need to standardize the amount of rapamycin given to dogs in the test group and have confidence that this standard dose will have the same physiologic consequences in all the dogs in the study.