Here, we present a detailed molecular characterization of the function and evolution of the 89K PAI in epidemic isolates of S. suis 2. We provide genetic evidence showing that an 89K-borne integrase mediates sitespecific excision and circularization of 89K, with contribution from an auxiliary protein named excisionase. The 89K extrachromosomal circular excision product is selftransmissible and can conjugally transfer via a plasmidlike process into S. suis recipient cells through the 89Kencoded GI-type T4SS transport conduit. Thus far, while mobilization of genetic elements by T4SSs has been well studied in Gram-negative bacteria, the transfer of Grampositive mobile genetic elements by T4SSs is currently limited to conjugative plasmids and transposons. Without a previous comprehensive study, we present the first demonstration of the horizontal transfer of a PAI mediated by a GI-type T4SS in a Gram-positive bacterium. We suggest that this new mobile element is responsible, at least in part, for the recent STSS outbreaks in China and recent sporadic cases in neighbouring countries and areas including Vietnam and Thailand (Holden et al., 2009) , and it will possibly spread further in the future.