All clinical trials for drugs aimed to stop diseases such as Parkinson’s disease and Alzheimer’s disease have failed, and currently there are no drugs that can stop or slow down these diseases. In parallel, we have gathered a lot of data on how genetics affect the risk for these diseases and their clinical progression. However, in most clinical trials, genetics is not being considered in the trial design. In this paper we simulate clinical trials for Parkinson’s disease and how genetic differences between the trial participants may affect trial results. We show that it is important to use genetic data in the trial design so that the genetic background of the participants will not affect trial outcomes. (By Dr. Hampton Leonard, https://jmg.bmj.com/content/early/2019/11/29/jmedgenet-2019-106283 )