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Fluorescence in-situ hybridization (FISH) is the method of choice for quantitative human epidermal growth factor receptor 2 (HER2) (also known as ERBB2) gene testing in invasive breast cancer. HER2 testing has great clinical impact, and is often claimed to expeditiously complete the entire diagnostic procedure for an individual patient. Against this background, the aim of this study was to evaluate the usefulness and performance of a novel dual-colour HER2/cen17 FISH assay designed to facilitate flexible (overnight) and rapid (<2 h of hybridization) FISH.We quantitatively and qualitatively compared counting results and the performance of the FlexISH SPEC ERBB2/CEN 17 dual-colour hybridization kit with well-established HER2FISH assays, by using 90 malignant (polysomic and non-polysomic) and 19 benign paraffin-embedded breast tissue specimens. We used long (overnight) and short (2 h) hybridization periods, and found an excellent correlation between the FISH results obtained with FlexISH, ZytoLight and PathVysion hybridization probes.The results obtained with both the short-run and long-run application of FlexISH are in excellent accordance with the results obtained with other commercially available FISH kits. This appears to be true in all relevant respects: signal counts, signal-to-noise ratio, brightness, and distinctness of HER2 and cen17 signals. As FlexISH probes can be equivalently used as a short-run or long-run application, the FlexISH probe kit provides the highest flexibility in terms of time and laboratory management. If required, a reliable HER2 finding can be delivered within 4.5 h, but the standard workflow (including overnight hybridization) does not negatively affect the performance, specimen quality or diagnostic result.