The oomycete pathogen Phytophthora infestans secretes cystatin-like effector proteins (EPICs) that inhibit secreted host proteases during infection. We recently found that the C14 protease is a relevant target of EPICs and that this protease is under diversifying selection in wild potato species with which P. infestans has coevolved. Here we generated a model of the EPIC-C14 complex based on cystatin-papain crystal structures and discovered three regions where variant residues in C14 might be the result of an arms race between enzyme and inhibitor at the plant-pathogen interface.