Baryons And Branes In Anti de Sitter Space
  In the mapping from four-dimensional gauge theories to string theory in $AdS$ space, many features of gauge theory can be described by branes wrapped in different ways on $\S^5$, $\RP^5$, or subspaces thereof. These include a baryon vertex coupling N external charges in the fundamental representation of SU(N), a bound state of $k$ gluons in SO(2k) gauge theory, strings coupled to external charges in the spinor representation of the gauge group, and domain walls across which the low energy gauge group changes. 
Edward Witten