In addition to our network of blood vessels, humans have a network of vessels known as lymphatic vessels. These vessels have a role in many processes in the body, including regulating fluid levels in tissues and immune surveillance. Although dysfunction in the lymphatic system contributes to human diseases such as the spread of cancer to other sites and lymphademas (localized fluid retention and tissue swelling), little is known about the molecules that regulate the formation of new lymphatic vessels, a process known as lymphangiogenesis. However, a team of researchers has now identified a role for the gene regulatory protein COUP-TFII in lymphangiogenesis in mouse embryonic development and tumor lymphangiogenesis in adult mice.