CASP9 (Total Importance: 12.49 in ALS, 9.80 in PD) was protective in ALS (OR: 0.854; 95% CI: 0.807–0.902; p = 2.41 × 10−8) but increased risk in PD (OR: 0.924; 95% CI: 0.855–0.998; p = 0.045), reflecting its role in protein homeostasis and apoptosis. This evidence concerns the gene CASP9 and Parkinson disease.