LY9 and breast cancer: This is the first study to indicate and validate the potential association of miR-542-5p and its downstream genes Tyrosine 3-Monooxygenase/Tryptophan 5-Monooxygenase Activation Protein Beta (YWHAB), Lymphocyte Antigen 9 (LY9), and Secreted Frizzled Related Protein 1 (SFRP1) with the prognosis of breast cancer.