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SaPIs (Staphylococcus aureus or superantigen pathogenicity islands) are a family of mobile genetic elements resident in the genome of some strains of Staphylococcus aureus. Much like bacteriophages, SaPIs can be transferred to uninfected cells and integrate into the host chromosome. Unlike the bacterial viruses, however, integrated SaPIs are mobilized by host infection with "helper" bacteriophages (specific SaPIs may require specific helper bacteriophages for mobilization, though Staphylococcus phage 80alpha appears to mobilize all known SaPIs).

One particular SaPI, SaPI1, is encapsidated in and exits the host cell in particles assembled from proteins encoded by its helper phage.