In three cases, drug costs are reduced by targeting the population with higher probability to respond to a specific treatment (HER2 testing for trastuzumab treatment in women with HER2-positive BrCa, EGFR mutation testing in NSCLC patients for EGFR-TKI treatment-decisions, and testing for KRAS mutation in CRC to target the use of cetuximab and panitumumab). The gene discussed is ERBB2; the disease is invasive breast carcinoma.