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A dimer consists of two monomer subunits bonded together. Homodimers are dimers where the two subunits are identical, whereas heterodimers are dimers where the two subunits are different from each other.
Dimers can consist of nucleic acid subunits. For example, undesired "primer dimers" can form when performing polymerase chain reactions for certain primer designs. However, in other situations, nucleic acid dimers may be desirable.