Replacement of a single residue in a β-hairpin by Gly-Pro, a 'united-residue' known to be overrepresented in protein β-sheets as Gly-cisPro without causing much structural distortion, and comparison of the resulting β-hairpin with those of other single/double residue substitutions show that Gly-Pro but not Pro-Gly is well accommodated in the isolated β-hairpin.