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In immune responses, activated T cells migrate to B-cell follicles and develop into follicular T-helper (T_FH) cells, a recently identified subset of CD4⁺ T cells specialized in providing help to B lymphocytes in the induction of germinal centres^,. Although Bcl6 has been shown to be essential in T_FH-cell function, it may not regulate the initial migration of T cells or the induction of the T_FH program, as exemplified by C-X-C chemokine receptor type 5 (CXCR5) upregulation. Here we show that expression of achaete-scute homologue 2 (Ascl2)—a basic helix–loop–helix (bHLH) transcription factor—is selectively upregulated in T_FH cells. Ectopic expression of Ascl2 upregulates CXCR5 but not Bcl6, and downregulates C-C chemokine receptor 7 (CCR7) expression in T cells in vitro, as well as accelerating T-cell migration to the follicles and T_FH-cell development in vivo in mice. Genome-wide analysis indicates that Ascl2 …