Cancer is a leading cause of death in the U.S. and the world, largely due to our inability to block the spread of disease (termed metastases). However, over the past several years the roles of recently discovered genes, called microRNAs, have been shown to play vital roles in controlling cancer growth and metastases. One group of these microRNAs, called the miR-200 family, has shown particular promise by blocking many critical functions known to drive cancer. Recently, we discovered that the miR-200 family could block the formation of new blood vessels inside tumors, which resulted in decreased metastases. Our proposal focuses on understanding how miR-200 blocks formation of blood vessels in cancer, and further explores the use of miR-200 delivery as a new therapeutic option to treat cancer.