A gene encoding the endogenous superantigen Mls(c), which deletes Tcrb-V3+ T cells in the NOD inbred mouse strain, was found to co-segregate with Mtv3 on chromosome 11. This identifies a fourth gene encoding a deletion ligand for Tcrb-V3+ T cells and extends recently published observations in support of the hypothesis that a number of endogenous superantigens are the products of Mtv proviruses.