BTK and hairy cell leukemia: The pathophysiology of HCL is now better understood, opening the way for new therapies including combined chemoimmunotherapy, immunotoxins targeting CD22 (moxetumomab pasudotox), BRAF inhibitors (vemurafenib, dabrafenib), MEK inhibitors (trametinib, cometinib), and BTK inhibitors (ibrutinib).