  We propose an action for a sine-Gordon-like theory, which reproduces the
classical equations of motion of the Green-Schwarz-Metsaev-Tseytlin superstring
on AdS(5) x S(5). The action is relativistically invariant. It is a
mass-deformed gauged WZW model for SO(4,1) x SO(5) / SO(4) x SO(4) interacting
with fermions.
