A gene‐encoding ligand for deletion of T cells bearing TcRVβ6 and Vβ8.1 cosegregates a new mammary tumor provirus locus, Mtv‐50 in NC mice. The sequence of the open reading frame (ORF) in the 3′ long terminal repeat (LTR) of Mtv‐50 was strikingly similar to those of Mtv‐7, Mtv‐43 and exogenous mouse mammary tumor virus (SW) with properties of minor lymphocyte stimulating antigen 1a. Consistent with previous reports, clonal deletion of mature thymocytes bearing TcRVβ6 was defective during the early postnatal period of mice carrying Mtv‐50. Appreciable levels of mRNA corresponding to common Mtv ORF and Mtv‐6 ORF were expressed in the neonatal thymus, while little, if any, mRNA corresponding to Mtv‐50 ORF was detected in the thymus at the early postnatal stage. Delay in expression of Mtv‐50 ORF during the postnatal period may be responsible for the failure of clonal deletion of Vβ6‐T cells in the early postnatal life of mice carrying Mtv‐50.
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