Overall, these findings show that circuit structure and function in LRRK2 p.G2019S carriers are already altered before symptom onset and, presumably, already reorganized during neuronal development and brain maturation, suggesting that LRRK2 p.G2019S carriers require compensatory mechanisms for normal cognitive function, which makes them potentially more susceptible to PD. This evidence concerns the gene LRRK2 and Parkinson disease.