Multiple images of QSOs are formed when the gravitational field of an intervening galaxy bends the trajectories of the light rays. The granulation of the lens galaxy mass distribution (either in stars or compact dark matter objects) could induce non correlated variability between different images of the lensed QSO (microlensing). The relevance of microlensing depends on the source size and, on light of some recent results, could affect to the Broad Emission Lines of QSOs. In this contribution we review the effects on the BLR of microlensing by an isolated microlens, a straight fold caustic and a caustic network.