Mls-1 is an endogenous superantigen that leads to in vivo deletion and in vitro stimulation of T cell receptor (TCR) V beta 6-, 7-, 8.1-, and 9-expressing cells. The MA/MyJ mouse deletes the identical set of TCR from its mature T cell repertoire; however, it does not contain Mtv-7, the murine mammary tumor provirus (MMTV), whose sag gene encodes Mls-1. Interestingly, the superantigen activity of this mouse strain segregates with a new mammary tumor provirus, Mtv-43, not seen in other inbred strains. The predicted amino acid sequence of the sag gene of Mtv-43 was compared with that of Mtv-7. Strikingly, the COOH terminus of the two molecules is very similar, while all other MMTV-encoded superantigens differ 100% in this segment.

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