Most internal organs consist of tubes lined by a single layer epithelial cells; these tubes usually have a characteristic length. For most organs, little is known about how the length of these tubes is controlled. For example, the small intestine of mammals has a defined length, though very little is known of about the mechanisms that control this length. If a portion of the small intestine is damaged due to disease or injury, either embryonically or postnatally, the length of the small intestine never regenerates. We have uncovered a portion of pathway that controls the length of the small intestine during embryonic development.