ERBB2 and head and neck squamous cell carcinoma: HER3 is a kinase-deficient receptor that is phosphorylated in a ligand (neuregulin-NRG/heregulin-HRG)-dependent manner through the formation of heterodimers with other HER receptors, and in HNSCC specifically by EGFR and HER2, both of which are widely expressed in this tumor type20,21,35, as judged by blocking antibody studies.