Relying solely on Next-Generation Sequencing (NGS) is insufficient for HER2 testing in biliary tract cancers. Data shows NGS misses up to 15% of patients with HER2 overexpression detected by immunohistochemistry (IHC). Performing both tests is essential to avoid denying patients effective targeted therapies.

The introduction of ADCs into frontline ovarian cancer treatment creates a new challenge: conflicting biomarkers. A patient's tumor might be positive for both HER2 (an ADC target) and a BRCA mutation (a PARP inhibitor target), forcing clinicians to choose between two effective targeted therapies without clear guidance.

There's a growing recognition that the molecular profile of a primary tumor can differ significantly from its metastases. To guide treatment more accurately, the preferred practice is to biopsy an accessible metastatic lesion when possible, as this better reflects the biology of the active disease being treated.

NGS testing is revealing that acquired HER2 kinase domain mutations, not amplifications, are an emerging resistance mechanism in ER+ lobular breast cancer. This creates a targetable population for HER2 TKIs like neratinib or tucatinib, offering a new line of targeted therapy.

An individual tumor can have hundreds of unique mutations, making it impossible to predict treatment response from a single genetic marker. This molecular chaos necessitates functional tests that measure a drug's actual effect on the patient's cells to determine the best therapy.

The same cancer-driving mutation behaves differently depending on the cell's internal "wiring." For example, a drug targeting a mutation works in melanoma but induces resistance in colorectal cancer due to a bypass pathway. This cellular context is why genetic data alone is insufficient.

In HER2-positive colorectal cancer, the choice of targeted therapy depends on RAS mutation status. The tucatinib/trastuzumab combination is effective only in RAS wild-type patients. In contrast, the antibody-drug conjugate trastuzumab deruxtecan (TDXD) shows efficacy regardless of whether a RAS mutation is present.

HER2 amplification is a primary resistance mechanism to anti-EGFR therapies in colorectal cancer. Therefore, oncologists should avoid using drugs like panitumumab or cetuximab in HER2-positive patients, even if they are RAS wild-type, as these patients experience rapid progression on such regimens.

In the increasingly common scenario of a patient with multiple positive biomarkers, a clear hierarchy exists for treatment decisions. Based on the robustness and maturity of clinical trial data, HER2-directed therapy is the top priority, followed by PD-L1 immunotherapy, with Claudin-18.2 targeting considered third.