There are HER2 isoforms, splice variants, and mutations in both human HER2-amplified cancers and in mouse Neu-driven tumors that show increased homodimerization and more aggressive biology (Hart et al., 2020; Siegel et al., 1994); however, HER3 remains an essential partner for tumorigenesis (Vaught et al., 2012). Here, ERBB3 is linked to cancer.