Our current study has identified several biological functions of miR-99a including reduction miR-99a expression in breast cancer tissues and cell lines, reduction of breast cancer cell viability and induction of apoptosis and inhibition of tumorigenesis in vivo after overexpression of miR-99a, as well as molecular mechanism of miR-99a-mediated inhibitory effect of mTOR in human breast cancer cells. Here, MTOR is linked to breast cancer.