In systems biology, perturbation is an alteration of the function of a biological system by external or internal means such as environmental stimuli, drug inhibition, and gene knockdown. It is an approach often adopted in systems biology. In reverse engineering of gene regulatory networks, when a few genes in a cellular system are systematically perturbed, responses such as transcript levels from other parts of the system can be recorded to reconstruct a regulatory network diagram that more clearly depicts the relationships between genes. Such systematic perturbations in cells can also help to identify drug mode of action. In addition, genetic perturbations such as gene deletion, gene overexpression, and insertion mutation can be used to determine the function of a gene through experimental techniques.