Frequent loss-of-function mutations in MLL4 (sometimes called MLL2) and its homolog MLL3 have been identified in developmental diseases such as Kabuki syndrome (Ng et al., 2010), congenital heart disease (Zaidi et al., 2013), and in cancers such as medulloblastoma (Parsons et al., 2011; Jones et al., 2012; Pugh et al., 2012), non-Hodgkin lymphomas (Morin et al., 2011; Pasqualucci et al., 2011), breast cancer (Ellis et al., 2012), and prostate cancer (Grasso et al., 2012). Here, KMT2C is linked to breast cancer.