The development of the humanized monoclonal antibody trastuzumab (Herceptin®) has become a model for successful development in targeted cancer therapy. Throughout all phases of the clinical development program, a clinical trials assay (CTA) was used for immunohistochemical detection of HER2 overexpression in the tumor specimens to identify patients with metastatic breast cancer who were eligible for treatment with trastuzumab. While the clinical phase III study was conducted with trastuzumab, a new simplified IHC assay (HercepTest™) was developed by Dako. Studies were performed in order to demonstrate that the HercepTest™ assay was concordant with the CTA and could be utilized as an aid in the assessment of patients for whom trastuzumab treatment should be considered. The final concordance study (n = 548) performed on a stratified sampled specimen, with a relative high number of 2+ cases, showed an agreement between the HercepTest™ and the CTA of 79% (95% CI: 76%-82%). The HercepTest™ was approved by FDA in September 1998 simultaneously with trastuzumab in a novel coordinated process between the Center for Drug Evaluation and Research and the Center for Devices and Radiological Health. The parallel drug-diagnostic 44development model used for trastuzumab by Genentech has become the standard for today’s development of targeted cancer drugs.