There is evidence to support a role for transforming growth factor-β in promoting their proliferation in tumours and in tumour-draining lymph nodes (TDLN)9,10 and also for driving conversion of conventional CD4+ Foxp3− T cells (Tconv cells) into CD4+ Foxp3+ Treg cells.11 In a previous study whereby the carcinogen methylcholanthrene (MCA) was used to induce fibrosarcomas in mice, we examined the T-cell receptor (TCR) repertoires of Tconv cells and Treg cells and found that the repertoires of tumour-infiltrating Tconv and Treg cells were distinct. Here, FOXP3 is linked to fibrosarcoma.