The process begins with the preparation of tissue sections, which are mounted on slides and treated to preserve the nucleic acids. A labeled DNA probe or RNA probe complementary to the target sequence is then applied. This probe hybridizes with the target nucleic acids within the tissue. After hybridization, a series of washing steps removes any unbound probe. The bound probe is detected using a chromogenic substrate that reacts with an enzyme conjugated to the probe, often horseradish peroxidase (HRP) or alkaline phosphatase (AP). This enzymatic reaction produces a colored precipitate at the site of hybridization, which can be observed under a light microscope.
