BTK and myeloid sarcoma: Through both spectrotyping and high‐throughput sequencing, increased usage of IGHV1 and IGHV4 families,69, 82, 83, 84 specifically IGHV1‐69, IGHV4‐34 and IGHV4‐39, have all been found in the brains of patients with MS.82 Overrepresentation of IGHD2, IGHD3 and IGHJ4 families has also been described.82 More recent studies using high‐throughput sequencing have shown B‐cells in the CSF of patients with MS are not representative of peripheral blood lymphocytes, indicating tissue‐specific clonal B‐cell expansion and localization.85