To characterize the effects of AGX51-mediated ID protein loss on cell growth, we determined the IC50 of AGX51 in 4T1 cells and nine other breast cancer cell lines representing the major breast cancer subtypes (ER+, HER2+, and TNBC), and three breast cancer PDX cell lines (Supplementary Table 1 and Supplementary Fig. 1f, g). Here, ID1 is linked to breast cancer.