ERBB3 and neoplasm: Although HPK is now confirmed to be HER3-specific, its initial evaluation for targeting HER2+ tumor cells [19,20,67,69] was based on the propensity for neuregulins to interact with HER2+ tumors through the heterodimerization of HER2/ErbB2 with other ErbB receptor family members including HER3 [106], while HER2 lacks a ligand binding domain of its own [107,108,109].