The bacteriophage λ int protein binds to and promotes polynucleotide strand exchange within specific DNA segments called attachment sites. Previous work strongly suggests that the specificity of int protein action is based, at least in part, on its ability to recognize nucleotide sequences in the attachment sites. We suggest that int protein also recognizes structural features of the attachment sites such as the twist and roll angles between adjacent base pairs. This proposal is based on statistical analysis of the predicted twist and roll angles of a large collection of secondary attachment sites. The analysis shows that the oscillation patterns of these parameters are conserved in regions where int proteins binds.