When naïve T cells are activated and clonally expand, the progeny differentiates into functionally distinct states or ‘subsets’. The differentiation state of a T cells is indicative of its functional and migratory properties, and T cell differentiation states are linked to clinical outcome, for example, after infection, adoptive T cell therapy or transplantation. It has, however, been difficult to translate T cell behavior across species. I will be presenting a practical strategy to facilitate two-way crosstalk between human and mouse settings, and propose a new conceptual framework to study T cell diversification.