3-amino-1,2,4-triazole (also called 3-AT) is a competitive inhibitor of the HIS3 gene product (imidazoleglycerol-phosphate dehydratase). Adding 3-AT to the media raises the stringency of the HIS3 selection. Frequently used in two-hybrid screens to eliminate background colonies.
Stored as a powder at -20. Working concentrations vary depending on the desired stringency, from 0.2 mM to 25mM.
Possible carcinogen. Use gloves, work in a hood.
References: Sigma product information