Four helix bundles are a common structural motif or architecture that has been extensively documented. They can be observed both independently and as components of larger folding units. The motif comprises four helices packed together in a variety of different ways, forming a hydrophobic core. In the most common bundles, the helices that are adjacent in the amino acid sequence are also adjacent in the three dimensional structure, forming the so-called up-down-up-down four helix bundle. The side chains of each helix in the bundle are arranged such that the hydrophobic side chains are buried between the helices and the hydrophilic side chains are exposed on the surface of the bundle. The example shown is a transferase (CATH domain ID 1tqgA00).