The primary structures of bovine and porcine pancreatic phospholipase A2 differ only by about 15%. Nevertheless, a 12 residue loop, with only one substitution (Val leads to Phe) has a quite different conformation, whereas the rest of the molecules have a very similar folding indeed. From this observation it is concluded that prediction of a 3-dimensional structure on the basis of sequence similarity of short segments alone might give erroneous results.