S tudies of modifier genes in cystic fibrosis (CF) have often been performed in small or narrowly defined populations, leading to conflicting results. In this issue of the JCI, Dorfman et al. demonstrate in a large, population-based study that two previously studied modifier genes, coding for mannose-binding lectin 2 and TGF-β1, influence pulmonary outcome in pediatric CF patients (see the related article beginning on page 1040). They further show gene-gene interaction between the two, underscoring the complexity of CF lung disease. Their findings provide further impetus to study these molecules and associated signaling pathways in CF. In addition, these findings argue strongly for collecting genotypes of known modifiers prospectively in CF clinical trials as well as in longitudinal studies of infants identified through newborn screening, where the full impact of such modifiers can be defined more precisely.