In addition to in vivo methods, a number of in vitro methods have become established in recent years for the analysis of signalling pathways. Reverse phase microarrays are one of the methods that are becoming more and more popular. Here the extract of cells e.g. from a biopsy or of cells with and without stimulus is spotted in a concentration series on slides and incubated with specific antibodies (e.g. phospho-specific antibodies). The signal intensity allows conclusions about changes in the proteins. Changes can be e.g. the abundance of the proteins or post-translational modifications. The advantages of microarrays are the small sample volume, a high number of technical replicates and the possibility to test a large number of samples under identical conditions. This allows the investigation of time courses as well as the analysis of signaling pathways or the "reaction" of cells (tissue) to external stimuli in order to draw conclusions about the mechanism of action.