 Ligand binding by the START domain in multidomain proteins can also regulate the activities of the other domains, such as the RhoGAP domain, the homeodomain and the thioesterase domain.The crystal structure of START domain of human MLN64 shows an alpha/beta fold built around a U-shaped incomplete beta-barrel.Most importantly, the interior of the protein encompasses a 26 × 12 × 11-Angstrom hydrophobic tunnel that is apparently large enough to bind a single cholesterol molecule.