BTK and mantle cell lymphoma: The first-in-class BTK inhibitor approved for use by the US Food and Drug Administration (FDA) was ibrutinib (PCI-32765, Imbruvica, Janssen), which was initially registered in November 2013 for relapsed/refractory (R/R) mantle cell lymphoma (MCL) and in February 2014 for R/R chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL) (17).