IRS1 and breast cancer: Just as an example, miR-126 inhibits cell cycle progression from G1/G0 to S phase of breast cancer (BC) cells by directly targeting the insulin receptor substrate 1 (IRS-1) constitutively activated in tumors [27], while it reduces trastuzumab resistance of BC cells by directly targeting phosphoinositide-3-kinase regulatory subunit 2 (PIK3R2, also known as p85β) [28].