This family of phage Cox proteins is expressed by Enterobacteria phages. The Cox protein is a 79-residue basic protein with a predicted strong helix-turn-helix DNA-binding motif. It inhibits integrative recombination and it activates site-specific excision of the HP1 genome from the Haemophilus influenzae chromosome, Hp1. Cox appears to function as a tetramer. Cox binding sites consist of two direct repeats of the consensus motif 5'-GGTMAWWWWA, one Cox tetramer binding to each motif. Cox binding interferes with the interaction of HP1 integrase with one of its binding sites, IBS5. This competition is central to directional control. Both Cox binding sites are needed for full inhibition of integration and for activating excision, because it plays a positive role in assembling the nucleoprotein complexes that produce excisive recombination, by inducing the formation of a critical conformation in those complexes.