PER2 and Parkinson disease: Figure 3 shows the network of functional interactions between the genes that were selected for this study (Hnmt, Hrh1, Hrh3, Per1, Per2, and Chrm3). This network shows that changes in the expression of the Hnmt, Hrh1, Hrh3, and Chrm3 genes can lead to changes in histamine and dopamine levels. In turn, changes in the levels of these neurotransmitters can influence the circadian genes Per1 and Per2, circadian rhythms, and PD.