Comparing DNA from patients at the best and worst extremes of a health condition can reveal genes for resistance and susceptibly. This approach discovered rare variations in the DCTN4 gene among cystic fibrosis patients prone to early, chronic airway infections. The gene codes for part of a molecular motor that moves microbes along a cellular conveyer belt to their annihilation. Similar “testing the extremes” strategies may uncover genes behind more common traits — healthy and unhealthy hearts, leaness and obesity, and normal and high blood pressure.