We then assessed the functional role of MLK4 by knocking-down MLK4 in several cell lines including breast cancer cell lines with high MLK4 expression (HCC1806, CAL-85-1, MDA-MB-436, ZR-75-1, and MCF7), low MLK4 expression (BT474) and in immortalized breast epithelial cells (MCF10A). Here, MAP3K21 is linked to breast cancer.