PDCD1 and myeloid sarcoma: Specifically, there is GWAS evidence supporting the relevance of T follicular helper cells (Tfh) in MS by identifying polymorphisms in the Tfh genes interleukin 21 (IL-21), C-X-C motif chemokine receptor 5 (CXCR5), and programmed cell death 1 (PD-1) associated with diagnosis or disease prognosis.