Members of this family are responsible for binding the DNA attachment sites at each end of the Mu genome. They adopt a secondary structure comprising a four helix bundle tightly packed around a hydrophobic core consisting of aliphatic and aromatic amino acid residues. Helices 1 and 2 are oriented antiparallel to each other. Helix 3 crosses helices 1 and 2 at angles of 60 and 120 degrees, respectively. Excluding the C-terminal helix 4, the fold of the I-gamma subdomain is remarkably similar to that of the homeodomain family of helix-turn-helix DNA-binding proteins, although their amino acid sequences are completely unrelated.