Plant pathogenesis-related proteins in class 10 (PR- 10) have well-defined structures , but the physiological function unclear . The discovery that the plant hormone binding protein from the group of cytokines ( CSBP ) belong to a group of structurally PR- 10 , suggesting that the binding of hydrophobic ligands of the general biological role of this entire class. This is confirmed by the structure of lupine protein complexes LLPR - 10.2B of cytokinins . It is surprising that these proteins are capable of binding in a vast cavity (1100-4500 3 ) not one but a number of molecules of the ligand, although the kinetic measurements suggest a 1:1 stoichiometry . It appears that the ability of PR- 10 proteins to bind a variety of ligands and, in different ways , mainly corresponds to the C-terminal helix a3 , which not only is characterized by high sequence variability , but plastic is also geometrically conforming to the bound partner. Surprising discoveries in the field of PR -10 proteins and their complexes make us re- look at the seemingly already well- developed aspects of molecular recognition and binding specificity of protein -ligand interactions .