CD44 and breast cancer: Notably, drug resistance- and CSC-associated proteins were identified exclusively in 4T1/EPB-derived UPs, including eight breast cancer-associated drug-resistant proteins: MDR1A, MDR1B, 4F2, GSTP1, GSTA4, MGST3, GSTO1, and GSTM2 (14); four breast cancer-associated stem cell proteins: CD44, integrin-α6, EpCAM, and ALDH1 (15, 29); and five therapeutic target-associated proteins: MAGED-1, SART3, CAV1, PCNA, and AKT1 (30, 31).