During positive selection, immature thymocytes commit to either the CD4+ or CD8+ T cell lineage (“commitment”) and convert from short-lived thymocytes into long-lived T cells (“rescue”). By formal precursor-progeny analysis, we now identify what is likely to be the initial positive selection step signaled by αβTCR, which we have termed “induction”. During induction, RAG mRNA expression is downregulated, but lineage commitment does not occur. Rather, lineage commitment (which depends upon the MHC class specificity of the αβTCR) only occurs after downregulation of RAG expression and the consequent fixation of αβTCR specificity. We propose that positive selection can be viewed as a sequence of increasingly selective developmental steps (induction→commitment→rescue) that are signaled by αβTCR engagements of intrathymic ligands.