Mice with reversible RAG1 deficiency (controlled by conditional Cre-recombinase expression) were crossed with TcrdH2BeGFP reporter mice (which allow the tracking of γδ T cells by green fluorescence) to monitor the de novo thymic development of γδ T cells. The authors found that interleukin-17 (IL-17)-producing γδ T cells are only generated in the embryonic thymus between E15.5 and E18.5 and that this process might be independent of T cell receptor rearrangement. IL-17-producing γδ T cells could not be reconstituted in lethally irradiated IL-17-deficient adult mice by transplantation of IL-17-sufficient bone marrow, and the authors showed that the development of these cells requires an embryonic thymic environment. These cells then persist as a long-lived, self-renewing population in adult mice. The development of IL-17-producing γδ T cells in adult thymi might be negatively regulated by IL-17 (produced by T helper 17 cells, γδ T cells and innate lymphoid cells).