Degree-dependent duplication (DDD) model. In the DDD model, the probability of a duplication of a node is dependent on the degree of the node. In the network at the left, node A is duplicated to generate node A' with the probability of (1 + 4σ)/1,000, because the degree of node A is four (see Methods). In the asymmetric divergence, each of the links to node A' is removed with a uniform probability α in the divergence process (top, second column). In the symmetric divergence, one of the two duplicated links (e.g. either A-B link or A'-B link) to each node connecting to A and A' (nodes B-E) is eliminated with a probability α (bottom, second column). A new link between nodes A and A' is attached with the probability proportional to the number of common neighbors (nN) shared by these nodes (third column). In this case, the probability is 2β, because these nodes share two common neighbors (nodes C and D).