Fontanilles et al. reported that MYD88 had a diagnostic sensitivity of 24% for PCNSL but a specificity of 100%.9 More recently, Ferreri et al. report a sensitivity of 94% and specificity of 98% for the MYD88 mutation status and IL-10 levels in the CSF.10 Given the specificity of MYD88 reported in the literature at the time the patient was diagnosed, she was treated with high-dose methotrexate and rituximab even though her brain biopsy was inconclusive. The gene discussed is IL10; the disease is primary central nervous system lymphoma.