Ibrutinib is a type of drug called a kinase inhibitor. It is believed to block a protein called Bruton's tyrosine kinase (BTK) that helps CLL cells live and grow. By blocking this, it is possible that the study drug will kill cancer cells or stop them from growing. Ibrutinib has been FDA approved for the treatment of CLL patients who have received at least one prior treatment; however, the FDA has not yet approved ibrutinib as the first treatment for previously untreated CLL. Therefore, ibrutinib is still considered to be study drug, which means it is still being studied.