We found in our study that the RALDH2 and RALDH3 genes, which encode main enzymes in the synthesis of RA, had significantly higher levels of transcription (from 4 to 250 times) in the NPC in both patients with PD compared to HD (Figure 6), which may be one of the reasons for the higher level of HOX gene expression. This evidence concerns the gene ALDH1A2 and Parkinson disease.