PTTG1 is a homolog of yeast securin proteins, which prevent separins from promoting sister chromatid separation. It is an anaphase-promoting complex (APC) substrate that associates with a separin until activation of the APC. The protein has transforming activity in vitro and tumorigenic activity in vivo, and is highly expressed in various tumors. This protein contains 2 PXXP motifs, which are required for its transforming and tumorigenic activities, as well as for its stimulation of basic fibroblast growth factor expression. It also contains a destruction box (D box) that is required for its degradation by the APC. The acidic C-terminal region of the protein can act as a transactivation domain. It is mainly a cytosolic protein, although it partially localizes in the nucleus.