In structural biology, a protein subunit is a single protein molecule that assembles (or "coassembles") with other protein molecules to form a protein complex. Some naturally occurring proteins have a relatively small number of subunits and therefore described as oligomeric, for example hemoglobin or DNA polymerase. Others may consist from a very large number of subunits and therefore described as multimeric, for example microtubules and other cytoskeleton proteins. The subunits of a multimeric protein may be identical, homologous or totally dissimilar and dedicated to disparate tasks.