Human RI complexed with human angiogenin or ribonuclease I forms a dimer with human RI-angiogenin/ribonuclease I. the entire molecule of human RI consists of only LRRs; all belong to RI- likease class. the 2 complexes are held together by many hydrogen bonds formed between the N-terminal ß-strands of the 2 human RI molecules, resulting in the formation of an antiparallel lß-sheet. the whole RI dimer adopts a right handed helix whose helical parameters are almost like those of its individual monomers. These two hRI molecules are related by an approximate two fold axis of rotation, nearly perpendicular to the common helical axis.