In vitro assays are laboratory tests conducted outside of a living organism, typically in a controlled environment such as a test tube or petri dish. These assays are used in various scientific fields, including biology, pharmacology, and toxicology, to study the effects of substances on biological systems. In vitro assays allow researchers to investigate the mechanisms of action of drugs or toxins, screen potential new therapies, and study cellular processes in a controlled and reproducible manner. These assays are often used as a preliminary step before conducting more complex and expensive in vivo experiments in animal models or clinical trials in human subjects. Overall, in vitro assays play a crucial role in advancing our understanding of biological processes and developing new treatments for various diseases.