End-of-study tumor ER protein levels were measured to understand whether the differential efficacy of camizestrant, fulvestrant, and elacestrant was due to the extent of protein degradation in vivo. Although in the CTC174 model—equally sensitive to camizestrant and fulvestrant—we observed similar ER degradation levels, in the ST941/HI model (camizestrant-sensitive/fulvestrant-insensitive), elacestrant degraded ER less than fulvestrant and camizestrant 4 and 24 hours after treatment (Supplementary Fig. S6H). This evidence concerns the gene ESR1 and neoplasm.