ERBB2 and ductal breast carcinoma in situ: Importantly, M1A‐ or M2A‐conditioned medium also induced filling of the lumen—recapitulating ductal carcinoma in situ—in a recently established model where myoepithelial and luminal cells, isolated from reduction mammoplasty, are grown in collagen and reform into bilayer structures (Fig 2G and H), a phenotype induced also by overexpression of HER2 (Carter et al, 2017).