Dimers of rare gases and rare gas-halogen dimers are the most spread and studied excimers. Under special conditions, most OH-containing molecules form dimers, e.g. the water dimer. Catechin monomers, dimers, and trimers (oligomers) are colorless. The histone octamer is formed by a central H3/H4 tetramer sandwiched between two H2A/H2B dimers. Unrepaired pyrimidine dimers in humans may lead to melanoma. Most protein dimers in biochemistry are not connected by covalent bonds. The H2A-H2B dimers and H3-H4 tetramer also show pseudodyad symmetry. Overexposure to sun leads to the formation of thymine dimers, a form of DNA damage. The absorbed energy can result in defects including pyrimidine dimers. Fibronectin matrix assembly begins when soluble, compact fibronectin dimers are secreted from cells, often fibroblasts. Four possible hNatA catalytic-auxiliary dimers are formed by these four proteins. Persistent carbenes exist in equilibrium with their respective dimers. In the system and reversibly bind together to form dimers. This preparation was largely free of dimers and polymers of prolactin. Other dimers and clusters exhibit exchange behaviour.