  The symmetric orbifold of $\mathbb{T}^4$ is the analogue of free SYM in four
dimensions, and its dual is described by a tensionless string propagating in
${\rm AdS}_3\times {\rm S}^3 \times \mathbb{T}^4$. In this paper we study the
deformation of this exact AdS/CFT duality away from the free point. On the
symmetric orbifold side this amounts to perturbing the theory by the exactly
marginal operator from the $2$-cycle twisted sector. We identify the
corresponding perturbation in the dual worldsheet description, and show that
the anomalous conformal dimensions of a number of symmetric orbifold currents
are correctly reproduced from this worldsheet perspective.
