ERBB2 and head and neck squamous cell carcinoma: Rather, treatment with the HER2-blocking mAb pertuzumab resulted in phospho-ErbB3 inhibition, indicating that HER2/ErbB3 heterodimers form preferentially, as described originally by Tzahar et al. [47] and the HER2 kinase primarily activates ErbB3 in HNSCC.