The HR1 repeat was first described as a three times repeated homology region of the N-terminal non-catalytic part of protein kinase PRK1(PKN). The first two of these repeats were later shown to bind the small G protein rho known to activate PKN in its GTP-bound form. Similar rho-binding domains also occur in a number of other protein kinases and in the rho-binding proteins rhophilin and rhotekin. Recently, the structure of the N-terminal HR1 repeat complexed with RhoA has been determined by X-ray crystallography. It forms an antiparallel coiled-coil fold termed an ACC finger.