Alkaline fixation drastically improves the signal of in situ hybridization.

In situ hybridization (ISH) is widely used to detect DNA and RNA sequences within the cell and tissue sections. The important step in performing this technique is tissue fixation. We investigated the influence of the pH of the fixative on the outcome of ISH. Our studies indicate that alkaline formaldehyde dramatically increases the ISH signal with RNA probes. The increase in signal was observed for detection of low as well as for high abundance messages. The sensitivity of the method was increased 5- to 6-fold.