1LTP:
reference: Model of lactose repressor core based on alignment with sugar-binding proteins is concordant with genetic and chemical data., Nichols JC, Vyas NK, Quiocho FA, Matthews KS, J Biol Chem 1993 Aug 15;268(23):17602-12. PMID: 8349639

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).