  We present a top-down string theory holographic model of strongly interacting
relativistic 2+1-dimensional fermions, paying careful attention to the discrete
symmetries of parity and time reversal invariance. Our construction is based on
probe $D7$-branes in $AdS_5 \times S^5$, stabilized by internal fluxes. We find
three solutions, a parity and time reversal invariant conformal field theory
which can be viewed as a particular deformation of Coulomb interacting
graphene, a parity and time reversal violating but gapless field theory and a
system with a parity and time reversal violating charge gap. We show that the
Chern-Simons-like electric response function, which is generated perturbatively
at one-loop order by parity violating fermions and which is protected by a
no-renormalization theorem at orders beyond one loop, indeed appears with the
correctly quantized coefficient in the charge gapped theory. In the gapless
parity violating solution, the Chern-Simons response function obtains quantum
corrections which we compute in the holographic theory.
