CISH is a method of analyzing HER2 that combines some of the best characteristics of both IHC analysis and genetic information. The tissue morphology context is preserved, while the gene copy number is quantified and the results are on a permanent slide that can be visualized by a standard light microscope.
CISH assays depend on a technique called subtractive hybridization, which uses a DNA probe visualized by a peroxidase reaction. CISH is a newer method that uses a chromogen-labeled probe which offers several advantages, including the ability to view the morphologic features of the cells using a conventional light microscope.
CISH differs from FISH in the method of probe detection. The detection uses a silver chromogen to form a silver precipitate in the nuclei and a single copy of the HER2 gene is visualized as a black dot. Following detection of HER2, a labeled chromosome 17 probe is detected using a red ISH detection kit (alkaline phosphatase) and fast red chromogen. The location of the deposition is at chromosome 17. HER2 and chromosome 17 sequences are present in every human cell and act as internal controls along with the with control slides. Enumeration and scoring are applied only to the invasive tumor areas and are similar in method to the scoring systems in place for FISH. Despite its status as a newer method, recent studies cite close to 100% concordance between CISH and FISH results.