Left (right) panel: cross section of $\mu^+$ ($\mu^-$) from W$^+$ (W$^-$) boson decays at backward and forward rapidities measured in p-Pb collisions at $\snn = 5.02$ TeV. The vertical error bars (open boxes) represent the statistical (systematic) uncertainties. The horizontal width of the boxes corresponds to the measured rapidity range. The results are compared with theoretical calculations [24,26] performed both with and without including the nuclear modification of the parton distribution functions. In the top panels, the calculations are shifted along the rapidity axis to improve the visibility. The middle (bottom) panel shows the data and pQCD (FEWZ) calculations divided by the pQCD (FEWZ) calculations without nuclear modification of the PDFs.