CDKN1A and Parkinson disease: Next, exploring the target genes associated with the enriched pathways, we found 4 target genes common to all pathways (CCND2, CDK6, CDNKN1A, and MDM2; Fig. 2d), including 3 molecules involved in cyclin-dependent pathways [CCND2 (Cyclin D2), CDK6 (Cyclin Dependent Kinase 6), CDNKN1A (cyclin dependent kinase inhibitor 1A)], which have a strong association with degeneration of dopaminergic neurons in PD.