T-cell differentiation involves instructive differentiation as well as selective expansion of differentiated cells. The cytokines critical for the differentiation of each lineage instruct activated CD4 T cells to express their master transcription factors, T-bet for Th1, GATA-3 for Th2 and RORγt for Th17, as well as other lineage specific factors, IL-12R for Th1, Gfi-1 for Th2 and IL-23R for Th17. In many instances, only a portion of cells expresses the indicated transcription factors and adopts the differentiated phenotype. Such differentiated cells express the factors that determine responsiveness to particular cytokines, IL-12 for Th1, IL-2 for Th2 and IL-23 for Th17 cells, thus leading to selective expansion of those differentiated cells.