PGP and breast cancer: Ubiquitin C-terminal hydrolase L1 (UCH-L1) has alsobeen reported as an EV-based predictive biomarker of chemoresistance in breast cancer.UCH-L1 overexpression has been reported to induce upregulation of P-gp levels through theMAPK/ERK signaling pathway, thereby enhancing an MDR phenotype in breast cancer.