 Domains are the fundamental units of tertiary structure, each domain containing an individual hydrophobic core built from secondary structural units connected by loop regions.The packing of the polypeptide is usually much tighter in the interior than the exterior of the domain producing a solid-like core and a fluid-like surface.Core residues are often conserved in a protein family, whereas the residues in loops are less conserved, unless they are involved in the protein's function.