ERBB2 and breast cancer: Human MDA-MB-231, Hs578T, and MDA-B02 breast cancer cells, which are ER- and PR-negative and do not bear an amplification of HER2 gene (referred to as triple-negative breast cancer cells), had higher cell surface expression levels of integrin α5β1 and higher ITGA5 protein levels than luminal A (T47D, MCF-7, BT-474) and HER2-expressing luminal B (SKBr3) breast cancer cell lines, as judged by flow cytometry and western blotting, respectively (Fig. S3A, B).