There are 33 invariant amino acid positions out of 132 positions in 42 investigated sequences of ribonucleases from a number of mammalian species and a reptile (snapping turtle, Chelydra serpentina). These invariant residues are unequally distributed over 3 different parts of the molecule. The lobe of the S-protein part of the molecule, which lacks one disulfide bridge and has two shortened loops in turtle ribonuclease, has the lowest percentage of invariant residues, although the active-site residue His 119 is located in this part.