ab and gd T cells have distinct roles in the immune system but arise from common precursors in the thymus. Many details about their development are incompletely understood; in particular, how and when the ab vs. gd fate decision occurs during human T cell development is still being debated. The Taghon lab recently demonstrated that thymocytes committing to the gd but not the ab lineage undergo pronounced changes in chromatin accessibility. This suggests that the epigenetic drivers responsible for lineage specification may differ between the two developmental trajectories, but owing to the limitations of bulk analysis approaches, the underlying molecular processes have yet to be elucidated. To overcome these constraints, the proposed project will employ single cell sequencing technologies to build a high-resolution multi-modal map of human ab and gd T cell differentiation. This will allow us to profile gene expression, chromatin accessibility, surface marker levels and TCR locus rearrangement events along the two developmental trajectories and help characterise lineage-specific changes in these modalities. We will use the integrated data set to identify transcription factors with potential roles in the fate decision process and employ an in vitro lineage tracing system to establish the ab-gd bifurcation point. The results of this study will provide unique insights into the molecular mechanisms underpinning human ab vs. gd T cell development at an unprecedented resolution.