SILCS: Site Identification by Ligand Competitive Saturation is a ligand or drug design program that calculates the probability that general classes or specific R-groups will bind to a target molecule. This allows a medicinal chemist to design molecules that have a higher probability of binding the target protein. This method was developed by Dr. Alex MacKerell, Professor and Director of the Computer Aided Drug Design center at the University of Maryland, School of Pharmacy.