MAPK1 and head and neck squamous cell carcinoma: Moreover, pharmacologic inhibition of MAPK1/3 activity by GDC-0994 has resulted in p-ErbB3 activation in MAPK-mutant HNSCC primary cultures and cell models, but not in WT counterparts, indicating that MAPK activating mutations can modulate p-ErbB3 levels in HNSCC.