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A rearranged TCR α transgene remains transcriptionally inactive in rag-2–/– thymocytes but can be induced by CD3-mediated signals with concomitant maturation of double-negative (DN) thymocytes to the CD4+CD8+ double-positive (DP) stage. Reciprocally, the same signals silence pre-TCRα (pTα) expression. In normal C57BL/6 thymocytes, TCRα expression is not detected in DN thymocytes while, in contrast, TCRβ expression is initiated at the most immature c-kit+CD44+CD25- stage and continues throughout thymocyte development. pTα expression is first detected at the intermediate c-kit±CD44+CD25+ DN stage, increases during transition to the more mature c-kit-CD44-CD25+ stage and is lost at the DP stage. Thus, although TCRβ and pTα expression are independent, the pre-TCR complex mediates signals controlling the appearance of αβ TCR through selective regulation of TCRα and pTα genes. [ABSTRACT FROM AUTHOR]