MAPK3 and breast cancer: Similarly, siRNA-mediated TYRO3 inhibition reduced ERK1/2 phosphorylation in 3 of 6 breast cancer cell lines tested and in 2 of the 3 sensitive lines, ERK1/2 phosphorylation was accompanied by decreased levels of cyclin D1 [37], a cell cycle regulatory protein that is induced downstream of ERK1/2 in numerous cell types [97,98,99,100].