Immune dysregulation, polyendocrinopathy and enteropathy with X-linked inheritance (IPEX) is a serious disease arising from mutations in FOXP3. This gene codifies for a transcription factor whose dysfunction results in hyperactivation of T cells. It is not clear, however, why an intermediate phenotype is not seen in heterozygous females, who are completely healthy. In order to address this question, we investigated X-chromosome inactivation in peripheral blood lymphocytes from a heterozygous female with a child affected by IPEX. No preferential inactivation was shown in freshly sorted CD4+, CD8+, CD19+ cells or in IL-2 cultured CD4 and CD8 T cells, indicating that peripheral blood lymphocytes in these women are randomly selected. Moreover, only one single FOXP3 transcript was expressed by CD4 T cell clones analysed by RT-PCR, confirming that this gene is subject to X- inactivation. We hypothesize that hyper-activation of T cell in carriers of FOXP3 mutations is regulated by the presence of normal regulatory T cells.