Among the genes that were found to be abundantly overexpressed in highly metastatic rat cell lines compared to poorly metastatic cell lines, we identified a completely novel complementary DNA (cDNA) without any homologous or related genes in the database in 1994. The full-length cDNA of this rat gene was cloned, sequenced, and named metastasis-associated gene 1 (mta1), and eventually, its human cDNA counterpart, MTA1, was also cloned and sequenced by our group. MTA1 has now been identified as one of the members of a gene family (MTA gene family) and the products of the MTA genes, the MTA proteins, are transcriptional co-regulators that function in histone deacetylation and nucleosome remodeling and have been found in nuclear histone remodeling complexes. Furthermore, MTA1 along with its protein product MTA1 has been repeatedly and independently reported to be overexpressed in a vast range of human cancers and cancer cell lines compared to non-cancerous tissues and cell lines. The expression levels of MTA1 correlate well with the malignant properties of human cancers, strongly suggesting that MTA1 and possibly other MTA proteins (and their genes) could be a new class of molecular targets for cancer diagnosis and therapy.