The selective formation of porous silicon in nanowires is observed in Si/Ge epitaxial layers along Ge layers grown by molecular beam epitaxy on a Si(100) substrate after metal-assisted chemical etching in aqueous HF-H2O2 solution. We assume that Ge layers serve as channels for a hole current out of the semiconductor to sustain the dissolution reaction. The tunnelling of holes through the potential barrier at the semiconductor surface is assumed to be the dominating mechanism of the hole transfer to the electrolyte.