The GAL4-UAS system is a biochemical method used to study gene expression and function in organisms such as the fruit fly. It has also been adapted to study receptor chemical-binding functions in vitro in cell culture. It was developed by Hitoshi Kakidani and Mark Ptashne, and Nicholas Webster and Pierre Chambon in 1988, then adapted by Andrea Brand and Norbert Perrimon in 1993 and is considered a powerful technique for studying the expression of genes. The system has two parts: the Gal4 gene, encoding the yeast transcription activator protein Gal4, and the UAS (Upstream Activation Sequence), an enhancer to which GAL4 specifically binds to activate gene transcription.