Every step of metastatisation requires a specific set of molecules, some of which may be common among different solid cancers, such as matrix metalloproteinase (MMP)-9 in tumour intravasation [78,79,80,81,82,83], and some others being specific for different cancers or type of metastasis, such as PTHrP for skeletal metastases [73,84]. The gene discussed is MMP9; the disease is cancer.