HR and breast cancer: In this way, we found that (i) MCF-7, MCF-7/ER(Y537S) and MCF-7/ER(D538G) (Fig. 6b), (ii) MCF-7-AR (Supplemental Fig. S6f), and (iii) T47D and T47D-AR (Supplemental Fig. S6g) CSCs are all sensitive to targeting with the anti-MUC1-C ADC, indicating that drug-resistant HR+/HER2− BC cells are potential candidates for treatment with this agent.