Loss-of-function mutations in the JNK signaling pathway (e.g. MAP3K1, MAP2K4, and MAP2K7) are implicated in the etiology of breast cancer (Banerji et al., 2012; Cancer Genome Atlas Network, 2012; Ciriello et al., 2015; Ellis et al., 2012; Kan et al., 2010; Nik-Zainal et al., 2016; Shah et al., 2012; Stephens et al., 2012; Wang et al., 2014). Here, MAPK8 is linked to breast cancer.