It is well established that 4T1 cells are particularly proneto develop resistance to doxorubicin, partly due to their abilityto upregulate drug efflux pumps such as P-glycoprotein (MDR1)., In contrast, the IC50 values of the PDX model employed in this studyare not available, but PDXs are generally considered to retain thechemosensitivity profile of the patient tumor. This difference may contribute to the relatively higherefficacy observed in the PDX setting. Here, ABCB1 is linked to neoplasm.