  I give a model-theoretic setting for the modular $j$ function and its
derivatives. These structures, here called $j$-fields, provide an adequate
setting for interpreting the Ax-Schanuel theorem for $j$ (Pila-Tsimerman 2015).
Following the ideas of M. Bays, J. Kirby and A.J. Wilkie for exponential
fields, I prove a generic transcendence property for the $j$ function.
