A new class of solvable irrelevant deformations of 2d QFTs have been uncovered recently. In this talk, I shall discuss modular properties of the deformed theories. This deformation turns out to be unique once we impose modular invariance and a one-to-one map between the spectra of the deformed and undeformed theories. The deformed CFTs (also in higher dimensions) have been proposed to be dual to gravity on AdS with a finite cut-off. As a verification of this proposal, we shall show how sphere partition functions of the deformed theories agree with holographic computations.