BTK and primary central nervous system lymphoma: A more powerful approach for the treatment of tumours addicted to chronic active BCR signaling caused by genetic mutations upstream of BTK, such as those targeting the My-T-BCR complex (e.g., ABC-DLBCL of the MCD type, primary central nervous system lymphoma (PCNSL), and Waldenström Macroglobulinemia (WM)) emerged from the development of the small molecule ibrutinib.