ROR1 and breast carcinoma: MCF7 cells were generated from breast ductal carcinoma cells and have been used for studying breast-cancer-cell biology.8,9,13 We found that these cells do not express ROR1, but could be made to express ROR1 upon stable transfection with a ROR1-expression vector, thus generating MCF7–ROR1 (Supplementary Fig. 4).8,9