Metastases are responsible for the vast majority of cancer-related deaths, but, despite intense efforts to understand their underlying mechanisms with the goal of uncovering effective therapeutic targets, treatment of metastatic cancer has progressed minimally. In this review, we examine the biological programs currently proposed to be key drivers of metastasis. On the basis of evidence from a growing body of research, we discuss to what extent the cellular and molecular mechanisms that are suggested to underlie cancer cell dissemination are specific to the metastatic process, as opposed to representing natural primary tumor progression. Our review highlights the contrast between the abundance of insight gained into the events that constitute the metastatic cascade and the paucity of therapeutic options.

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