ERBB2 and cancer: This finding was further confirmed using the more comprehensive ErbB model.21 ErbB3 does not meet the conventional criteria of a drug target in cancer: it is generally expressed at low levels, is not gene amplified, is rarely mutated, and has very weak kinase activity.22 EGFR and HER2, which are considered validated drug targets, were also identified as sensitive targets for Akt phosphorylation, but their sensitivity is more dependent on the identity of the ligand driving pathway activation.