<P> Metastatic cancers may be found at the same time as the primary tumor, or months or years later . When a second tumor is found in a patient that has been treated for cancer in the past, it is more often a metastasis than another primary tumor . </P> <P> It was previously thought that most cancer cells have a low metastatic potential and that there are rare cells that develop the ability to metastasize through the development of somatic mutations . According to this theory, diagnosis of metastatic cancers is only possible after the event of metastasis . Traditional means of diagnosing cancer (e.g. a biopsy) would only investigate a subpopulation of the cancer cells and would very likely not sample from the subpopulation with metastatic potential . </P> <P> The somatic mutation theory of metastasis development has not been substantiated in human cancers . Rather, it seems that the genetic state of the primary tumor reflects the ability of that cancer to metastasize . Research comparing gene expression between primary and metastatic adenocarcinomas identified a subset of genes whose expression could distinguish primary tumors from metastatic tumors, dubbed a "metastatic signature ." Up - regulated genes in the signature include: SNRPF, HNRPAB, DHPS and securin . Actin, myosin and MHC class II down - regulation was also associated with the signature . Additionally, the metastatic - associated expression of these genes was also observed in some primary tumors, indicating that cells with the potential to metastasize could be identified concurrently with diagnosis of the primary tumor . </P> <P> Expression of this metastatic signature has been correlated with a poor prognosis and has been shown to be consistent in several types of cancer . Prognosis was shown to be worse for individuals whose primary tumors expressed the metastatic signature . Additionally, the expression of these metastatic - associated genes was shown to apply to other cancer types in addition to adenocarcinoma . Metastases of breast cancer, medulloblastoma and prostate cancer all had similar expression patterns of these metastasis - associated genes . </P>

In order for a malignant cancer to metastasize it must