The holographic dual to string theory in $AdS_3 x N$ has always been a fundamental question in high-energy theoretical physics. To this day, we don't know the answer to this question in full generality. In this talk, I'll propose an effective holographic dual to type IIB string theory in $AdS_3 x N$ in the presence of pure NS-NS flux. The dual boundary CFT takes the form of a p-fold symmetric product of $(R_\phi \times N)$ deformed by a $\phi$-dependent $Z_2$-twisted marginal operator. I'll explain how an exact worldsheet computation allows us to identify this marginal operator.

When the radius of $AdS_3$, $R_{ads}$, is sub-stringy, the CFT spectrum doesn't contain neither a normalizable vacuum nor the BTZ black hole states. The proposed holographic duality in this case is an exact one. On the other hand, when $R_{ads}/l_s >1$, the full boundary CFT does have a normalizable vacuum and the BTZ black hole states at high energies. The proposed duality in this case is an effective one and holds only for the perturbative string states in the spectrum. Finally, I'll quote some of the checks that have been performed to test this duality.