The cystatins are a family of cysteine protease inhibitors which share a sequence homology and a common tertiary structure of an alpha helix lying on top of an anti-parallel beta sheet.

Cystatins show similarity to fetuins, kininogens, histidine-rich glycoproteins and cystatin-related proteins.

Phytocystatins are involved in several process, including plant germination and defense.

van Wyk et al. found some 19 different cystatins similar to oryzacystatin-I in the soybean along with related cysteine proteases.

Harnessing the knowledge of cystatins in plants could contribute to the development of crops with improved resistance to pests and diseases.