A sparse gene regulation network and their structural identifiability properties. (a) This subfigure is the reproduction of one of the sparse gene regulation networks used for structural identification in . The network has 11 activation (solid edges), 6 suppression (dashed edges), and 3 autoregulation links (at nodes 1, 3 and 6) with undefined sign. (b) The subfigure shows that the vast majority of the randomly generated models in the case when N = 2 and K = 2 are structurally unidentifiable, because not all nodes of the network are reachable from the perturbed one. (c) As the network becomes less sparse (N = 10, K = 3), the structural identifiability properties are quickly improving. In this case, more than 80% of the randomly generated models are structurally identifiable.