CXCL13 and primary central nervous system lymphoma: The CSF levels of the following biomarkers have been shown to be elevated in CNS lymphoma: sCD27,14, 15 sCD19,16 β2‐MG,17 antithrombin III,18 free light‐chain immunoglobulins,19 a specific set of microRNAs,20, 21 IL‐10 9, 22 sIL2‐R,9 CXCL13,10, 23 and neopterin.24