SMAD4 and breast cancer: For example, miR-421 (targets: FXR, DPC4/SMAD4, ATM), miR-503 (target: CCND1), miR-24-3p (targets: p27, p16), miR-29a (targets: PTEN, GSK3β, TET1) and miR-451 (targets: MIF, YWHAZ) play a critical role in the proliferation, migration, invasion, and metastasis of BC.