The majority of cancer deaths are attributable to invasion and metastasis. Metastasis is the spread of disease from one organ or part to another not directly connected with the primary site; the defining characteristic of a malignant neoplasm. How this spread is achieved is important for understanding neoplastic disease. Characterisation of the molecules driving tumour dissemination may provide possible ways to block the process while, if secondary deposits continue to express the gene products which facilitated their dissemination, such products could also serve as potential therapeutic targets.