CXCL12 is elevated in the blood of PwPD56, and two independent studies have reported increased CXCR4 and CXCL12 expression in post-mortem SN tissue from PwPD57,58 These findings suggest that CXCR4-mediated immune cell trafficking may contribute to PD progression, and the role of M-MDSCs warrants further investigation. Here, CXCR4 is linked to Parkinson disease.