Abstract: | Summary Structural similarities between proteins with no amino acid sequence homology either indicate a phylogenetic relationship, or they are merely the expression of a physically preferred way of folding a polypeptide chain. It is shown that one can distinguish between these alternatives by evaluating the significance of the similarity . Such significances have been derived for comparison between chain folds containing -pleated sheets (Schulz and Schirmer, 1974; Richardson et al., 1976; Sternberg and Thornton, 1976). An extension of this method to comparisons between any two chain folds is outlined here. |