The Food and Drug Administration is proposing a system for approving customized drugs and medical treatments for patients with rare or hard-to-treat diseases. The pathway laid out Monday is aimed to approving one-of-a-kind therapies, including those using emerging gene-editing technology. It's a shift long sought by patients, advocates and researchers focused on rare diseases, which often do not fit within the pharmaceutical industry's business model. For many rare diseases, drug companies have had little financial incentive to develop new treatments. The FDA proposal, if finalized, would codify a path for drugmakers and researchers to study and commercialize treatments without conducting large, expensive clinical trials.