CCND1 and Parkinson disease: Cluster 7 cells displayed the most marked upregulation of known senescence markers, including CDKN2A, CCND1, and CCND2 mRNAs, as well as PINK1 mRNA, encoding the protein PINK, which is implicated in Parkinson’s disease and regulates mitochondrial quality control [19], and IGFBP5 mRNA, encoding a potent inducer of fibrosis and ECM remodeling [20].