Approximately two-thirds of human cancers, including skin, colon, lung, and pancreas; multiple myeloma; and hairy cell leukemia, have aberrations in the ERK1/2 cascade, largely due to activating mutations in signaling intermediates such as EGFR, KRAS, or BRAF (Davies et al. 2002; Garnett and Marais 2004; Dhillon et al. 2007; Bryant et al. 2014). Here, MAPK3 is linked to cancer.