Metastasis is the multi-step process by which tumor cells spread from a primary site to distant organs and establish secondary tumors. Because metastases are difficult to detect and cure, they are the cause of approximately 90% of cancer mortality. To elucidate the molecular mechanisms of tumor metastasis, it is essential to identify the genes and the resulting pathways responsible for individual steps of metastasis. Therefore, I have chosen to investigate an experimental murine mammary tumor metastasis system. This system is composed of four distinct cell populations, all originating from a single primary mammary tumor; each can complete a defined subset of metastatic steps. By comparing the differences in the gene expression profiles between the tumors formed by these four cell lines using DNA microarray analysis, I have identified several candidate genes whose expression strongly correlates with their corresponding metastatic ability. In this proposal, I plan to examine the roles of one candidate gene, Twist, play during mammary tumor metastasis. The specific aims of this proposal are: I. Identify candidate genes involved in individual steps of tumor metastasis. II. Elucidate the molecular mechanism of one identified gene, Twist, in promoting tumor metastasis. III. Examine the requirement of Twist during tumor invasion and metastasis in vivo.