 its own HIS3 analogue is not present or nonfunctional), an increased level of HIS3 expression is required in order for the yeast cell to survive.This has proved useful in various two-hybrid system, where a high-affinity binding between two proteins (i.e., higher expression of the HIS3 gene) will allow the yeast cell to survive in media containing higher concentrations of 3-AT.This selection process is performed using selective media, containing no histidine.