Integrase-mediated precise excision of mobile elements from the chromosome typically represents the first step in the lateral transfer process. Such excision events usually lead to the formation of circular episomal products that are intermediates in the packaging of some ICEs (e.g. P4) into phage particles (Bowden and Modrich, 1985) , or serve as substrates for conjugative transfer of other elements, such as PAPI-1 (Qiu et al., 2006) and SXT (Burrus and Waldor, 2003) . Sequence analysis revealed that 89K is not similar to an integrated prophage, suggesting that the excised circular 89K may undergo lateral transfer. Confirmation of this hypothesis was obtained by the observation that transfer of a Spc R -marked 89K occurred at a frequency of approximately 10 -6 per donor, similar to that of the Pseudomonas clc element (Sentchilo et al., 2003) and the Salmonella SGI1 island (Doublet et al., 2005) . Further, 89K transfer absolutely requires the 89K-borne integrase, while a Dxis derivative can still transfer at a low but measurable frequency. This suggests that 89K excision is required for its transfer, and Xis may exert its effect by reducing the number of copies of excised circularized 89K molecules. Notably, Gilmore and co-workers demonstrated that transfer of the enterococcal PAI did not require the PAI-resident integrase, excisionase or putative conjugation genes. Instead, transfer only occurred from donors possessing a pheromone responsive-type of conjugative plasmid via an Hfr-like and RecA-dependent mechanism (Manson et al., 2010) .