Bruton tyrosine kinase (Btk) is a member of the Tec family non-membrane tyrosine kinases that predominantly resides in the cytoplasm but gets recruited to the plasma membrane upon stimulation by B-cell receptor signaling. Btk is an enzyme that is crucial in multiple signal transduction pathways regulating activation, differentiation, survival and proliferation of B-lineage lymphoid cells. Loss of Btk causes X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice – conditions characterized by drastically reduced generation of mature B-cells and immunoglubulins to participate in the humoral immune response during infections. However, excessive Btk activation is associated with multiple B-cell malignancies. As such, research groups have been developing inhibitors (e.g. ibrutinib) that target Btk to serve as new treatment option for B-cell leukemias such as acute lymphoblastic leukemia (ALL) and chronic lymphocytic leukemia (CLL). The main objective of this project is to develop a specific chemosensor peptide substrate to monitor the Btk kinase activity in leukemic and bone marrow cells obtained from ALL and CLL patients before and after treatment with ibrutinib. Specifically, our group aimed to (i) use the arrayed combinatorial peptide library screen to define the optimal phosphorylation sequence of Btk 1, (ii) use the optimal phosphorylation sequence to develop a chemosensor peptide substrate for Btk 2 and (iii) perform routine clinical diagnostic analysis using the chemosensor peptide substrate to monitor the kinase activity of Btk in patient samples to evaluate the efficacy of ibrutinib as a treatment option for ALL and CLL as well as to facilitate identification of ibrutinib-resistant cases.