Multiple images of quasars formed by gravitational lensing are typically seen through the lensing galaxy. The granulation of the galaxy in stars or other compact objects induces local (`microscopic') but strong fluctuations in the gravitational potential that subdivide each image in several micro-images changing the expected flux of the image (flux anomalies). This phenomenon, quasar microlensing, allows to study both the unresolved structure of the source and the composition of the lensing galaxy. We have applied a statistical analysis based on caustic crossing to the multiply imaged quasar QSO B2237+0305. The results indicate that the abundance of low-mass stars in the distribution of microlenses could be estimated with 10 years of monitoring.