Our study shows that, during a relapse, CD39 ecto-ATPase activity is increased in RR-MS patients, independently of immunomodulatory treatment status, and the CD39 levels are increased specifically in circulating CD4+ CD25high CD127low FoxP3+ Treg cells from RR-MS patients compared with sex- and age-paired heathy subjects, especially in treated RR-MS patients. The gene discussed is FOXP3; the disease is myeloid sarcoma.