ROR1 and breast cancer: Recent studies have also shown that acquired resistance to T-DM1 (trastuzumab emtansine, a newly developed antibody-drug conjugate) in HER2+ breast cancer patients induced high ROR1 expression on neoplastic cells that exhibit stem cell signatures (such as expression of both CD44 and ALDH1), resulting in an enhanced capacity to form spheroids and self-renewal properties [21].