A survey of known protein structures reveals that approximately 70% of serine residues and at least 85% (potentially 100%) of threonine residues in helices make hydrogen bonds to carbonyl oxygen atoms in the preceding turn of the helix. The high frequency of intrahelical hydrogen bonding is of particular significance for intrinsic membrane-bound proteins that form transmembrane helices. Hydrogen bonding within a helix provides a way for serine, threonine and cysteine residues to satisfy their hydrogen-bonding potential permitting such residues to occur in helices buried within a hydrophobic milieu.