While this classical model of PTBP1 down-regulation and neuronal differentiation implies negligible expression or importance of PTBP1 in mature neurons, our results indicate that PTBP1 is sufficiently expressed and plays a functional role in mature neurons, with the installation of a loss-of-function mutation resulting in a detectable reduction in relative protein amounts (Figure 2—figure supplement 5; Figure 3—figure supplement 2) and behavioral improvements in a PD mouse model (Figure 5; Figure 5—figure supplement 1). This evidence concerns the gene PTBP1 and Parkinson disease.