At some point in fetal development, cells from the newly emerged blood circulatory system start out on their own and form a separate parallel network of vessels known as the lymphatic system. In the January 10th issue of Science, researchers report the discovery of the molecular signals necessary to separate the lymph vessel network from the blood vessel network. Their findings clarify an important juncture in fetal development, shed light on the mechanisms by which molecular signals influence vascular development, pave the way for potential therapeutics, and may ultimately clear up a minor mystery among researchers that has been brewing since the mid-1990s.