In T cells, two members of the Dok family, Dok-1 and Dok-2, are predominantly expressed. Recent evidence suggests that they play a negative role in T-cell signaling. In order to define whether Dok proteins regulate T-cell development, we have generated transgenic mice overexpressing Dok-1 in thymocytes and peripheral T cells. We show that overexpression of Dok-1 retards the transition from the CD4 − CD8 − to CD4 + CD8 + stage. Moreover, there is a specific expansion of PLZF-expressing Vγ1.1 + Vδ6.3 + T cells. This subset of γδ T cells acquires innate characteristics including rapid IL-4 production following stimulation and requiring SLAM-associated adaptor protein (SAP) for their development. Moreover, Dok-1 overexpression promotes the generation of an innate-like CD8 + T-cell population that expresses Eomesodermin. Altogether, these findings identify a novel role for Dok-1 in the regulation of thymic differentiation and in particular, in the development of PLZF + γδ T cells.Keywords: γδ T cell r Dok r PLZF r SAP r Thymocyte development Supporting Information available online Introduction αβ T-lymphocyte development is dependent on productive rearrangement of the TCRβ locus and intracellular signaling initiated through the pre-TCR at the CD4 − CD8 − double-negative (DN) stage. Pre-TCR signaling leads to cellular proliferation and progression to the CD4 + CD8 + double-positive (DP) stage where TCRα locus rearrangement is induced. In DP thymocytes, initial TCR signal strength/duration will determine specific gene expression and Correspondence: Dr. Pascale Duplay e-mail: pascale.duplay@iaf.inrs.ca as a consequence will regulate the outcome of thymocyte positive versus negative selection and CD4 versus CD8 lineage differentiation or conventional versus innate lymphocyte lineage choice [1].γδ TCR lymphocytes also develop in the thymus and require productive rearrangement of the TCRγ and δ loci at the DN stage. The strength of TCR signaling is important in controlling αβ/γδ lineage choice. Strong TCR signaling is required for the commitment to the γδ lineage whereas weak TCR signals favor * These authors contributed equally to this work. Eur. J. Immunol. 2012Immunol. . 42: 2491Immunol. -2504 development of αβ lineage cells (reviewed in [2][3][4]). Moreover, TCR signal strength regulates also maturation and acquisition of effector functions in γδ and αβ T-cell development. Therefore, signaling molecules that modulate TCR signal strength play a pivotal role in the regulation of T-cell development and homeostasis. Development signals from the pre-TCR and from the TCR are controlled by the coordinated activity of tyrosine kinases such as Syk-family kinases [5], Src-family kinases [6], and Tec-family kinases [7,8] and adaptor molecules such as , LAT [10], Grb2 [11], and Gads [12].Dok are adaptor proteins that contain a pleckstrin homology domain, a phosphotyrosine binding domain, and a COOHterminal region containing multiple tyrosine phosphorylation sites [13,14]. Dok-1 and Dok-2, the two members of Dok family proteins e...