ESR1 and breast cancer: Here,we explored the use of functionalized nanoparticles for the detectionof estrogen receptor alpha (ERα) expression in a 3D tumor model,using the ERα-positive human breast cancer cell line MCF-7.This approach was used to compare targeted versus nontargeted nanoparticleinteractions with the tumor model to better understand whether targetednanotags are required to efficiently target ERα.