  We point out that there is a difference between the behavior of fermionic
systems (and their holographic analogs) in a background axial vector field, on
the one hand, and at finite chiral chemical potential, on the other. In the
former case, the electric current induced by constant background axial field
$A_0$ and magnetic field ${\bf B}$ vanishes, while in the latter it is given by
the anomaly-prescribed formula ${\bf j} = \frac{\mu_A}{2\pi^2}e^2 N_c {\bf B}$.
