In recent years, Tyramide Signal Amplification (TSA) has gained acclaim as a very sensitive detection method for immunocytochemsitry and fluorescence in situ hybridization (FISH). To maximally exploit the great signal generation capacity of TSA in mRNA-FISH, minimizing signals emanating from non-specifically bound nucleic acid probe becomes of prime importance, because a specificity check of the signals observed in the cytoplasm is virtually impossible. We reasoned that utilization of synthetic oligonucleotides (ONTs) in stead of commonly used cDNAs or cRNAs would diminish non-specific probe binding and that direct Horse Radish Peroxidase (HRP) labelling of ONTs and TSA would enable their in situ detection.

This approach was first tested in metaphase DNA-FISH using chromosome-specific repeats as targets. Using bifunctional crosslinking chemistry and HPLC, 5’-hexylamino oligonucleotides for chromosome specific simple satellite and alphoid sequences were conjugated to HRP and purified. Following 15 - 20 min of situ hybridization of a single HRP-ONT probe to metaphase chromosomes and a direct flurochrome-tyramide detection step, such repeat targets could be visualized with high specificity and excellent signal-to noise ratio.