ERBB4 and breast cancer: For instance, ERBB4/HER4 is a PPxY motif‐containing ITCH substrate (Omerovic et al, 2007); transgenic mice expressing the JM‐a/CYT‐1 isoform of the erbb4 gene develop mammary tumors (Wali et al, 2014), and sustained expression of ERBB4 activates the MAPK ERK1/2 pathway in response to its ligand heregulin (Vidal et al, 2005).