conjecture though.

QCD vs SYM 
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It would be useful if the four dimensional theory on the boundary were QCD, since this would allow us to explore its non-perturbative regime by studying a perturbative dual theory. However, the field theory described by the correspondence is a supersymmetric theory with conformal invariance, while QCD has none of these features. The most important differences between the two theories are\[63\]:

-   QCD confines while SYM is not confining.

-   QCD has a chiral condensate while SYM has no chiral condensate.

-   QCD has a discrete spectrum while that of SYM is continuous.

-   QCD has a running coupling while SYM has a tunable coupling and is conformal.

-   QCD has quarks while SYM has adjoint matter.

-   QCD is not supersymmetric while SYM is maximally supersymmetric.

-   QCD has $N_c = 3$ in real life, while the AdS/CFT correspondence holds for large $N_c$.

However, the gauge/gravity duality can be expanded to more field theories by changing the supergravity theory. This gives a possibility to search for a field theory that is closer to QCD and has a gravity dual.

-   For example considering multiple D3-branes on curved backgrounds, leads to an interesting family 