The alternating decision tree. The alternating decision tree used to classify instances. A gene is classified with the tree by beginning at the node marked "Start" and then following each branch in turn. Upon reaching a node which contains an assumption the "yes" or "no" branch is followed as appropriate. If the relevant feature is "unknown", neither branch is followed. Adding up each of the numbers in rectangles that are encountered along the way results in a final score which reflects the relative confidence of the classification. The classification itself is based on the sign of the score.