A unit comprising two or more iron atoms and bridging sulfur ligands in an iron-sulfur protein. The recommended designation of a cluster consists of the iron and sulfur content, in square brackets, for example, $\ce{[2\! Fe-2\! S]}$, $\ce{[3\! Fe-4\! S]}$. The possible oxidation levels are indicated by the net charge excluding the ligands, for example $\ce{[4\! Fe-4\! S]^{2+}}$; $\ce{[4\! Fe-4\! S]^{+}}$ or ($\ce{[4\! Fe-4\! S]^{2+;1+}}$) cluster.