The search tree is built up from nodes that contain states with current molecules of the route. The root node contains one molecule, which is the target molecule of the algorithm. Other nodes can contain states with one or more molecules. Each node is bound to others in a directed way as parent-child nodes, with actions as edges. The action is the retrosynthetic reaction performed on one of the molecules of the parent state, to yield the molecules of the child node state. The search algorithm starts with the expansion of the root node (see below).