The K_d fit model describes a molecular interaction with a 1:1 stoichiometry according to the law of mass action. Interactions where several molecules A bind to one molecule B, but all binding sites have the same affinity for A can still be described by this model because the binding events are independent. DI.Screening Analysis software offers two fitting models: the K_d model and the Hill model. The K_d model is suitable for most investigated interactions. Use the Hill model only for data evaluation if the investigated interaction is known to be cooperative.