Infection with Pseudomonas aeruginosa is a common complication in patients with cystic fibrosis. Emond et al. carried out exome sequencing of 91 patients who represented either end of the phenotypic spectrum of infection severity. They found variants in dynactin 4 (DCTN4) that correlated with earlier and more severe infection, and these were validated by targeted resequencing of an independent cohort. This demonstrates the proof of principle that exome sequencing and an extreme phenotype design can identify complex trait genes.