RB1 and cancer: In addition, the levels of phosphorylated retinoblastoma (pRB, a downstream effector of cyclin D1) [21] and matrix metalloproteinase 9 (MMP-9, a downstream gene of Ets1) [22] were decreased in miR-9 over-expressing cancer cells (Fig. 2B, Fig. 2C, and Fig. 2D).