A recent innovative study in SPMS patients confirmed a role for the gut–brain axis in MS patients, showing the depletion of a subset of circulating memory CD4+ T cells expressing the gut-homing chemokine receptor CCR9 and the α4β7 adhesion molecule and a tendency to switch from a regulatory to a pro-inflammatory phenotype that produces more IFN-γ and IL-17 (213). The gene discussed is IL17A; the disease is myeloid sarcoma.