Knockdown of CCR3 or blockade of its ligand CCL5 can partially suppress MSC migration, suggesting that the CCL5-CCR3 axis may promote MSC homing.73 Treatment with the cationic molecule polyethylenimine (PEI) dose-dependently increased CCR4 expression on MSCs, and the brain homing efficiency of MSCs significantly increased after intravenous administration of PEI in a rat model of brain injury.74 This evidence concerns the gene CCL5 and brain injury.