Precision medicine is founded on the basis of therapies that selectively engage proteins in diseased tissue while avoiding healthy tissue. This approach is most powerfully demonstrated by drugs like imatinib and vemurafenib. These drugs exploit changes in ligand binding sites that are modified by mutation. Although undoubtedly successful, this approach does not fully exploit the differential expression of proteoforms in disease states. We are developing improved methods for measuring proteoform-specific interactions of small-molecule inhibitors and exploring new chemical matter that can achieve selective inhibition.