What is it? The INFORM HER2 Dual ISH DNA Probe Cocktail is a laboratory test that uses DNA probes, with color-forming (chromogenic) dyes attached, to count how many copies of the HER2 gene, located on chromosome 17, are present in a tissue sample. This test, which uses a variation of fluorescence in situ hybridization (ISH), is based on color detection, rather than fluorescence, and is used with samples of tumor tissue removed from breast cancer patients. This test is used by a doctor (pathologist) trained to identify diseases by studying slices of cells and tissues placed on microscope slides. If the test result indicates that too many copies of the HER2 gene (gene amplification) are present in the cancer cell, then a breast cancer patient may be eligible for treatment with the anti-cancer drug Herceptin® (Trastuzumab).