ERBB4 and neoplasm: The isoform β2 of the heregulin (HRG) family of growth factors (HRGβ2) is a tumor-promoting growth factor conventionally described as an indirect activator of HER2 (erbB2) oncogene-driven signaling via its ability to bind HER3 (erbB3) and HER4 (erbB4) [1–8].