Additional Study Information: Why is this study being done? This research is looking at a possible new treatment for people who have ongoing pain from a condition called small fiber neuropathy and haven't found relief from other treatments. How does the study work? This is a double-blind study. That means you won't know whether you are getting the real treatment (ST-503) or a sham (a fake treatment with no effect), and most of the study team won't know either. This helps make the results fair and reliable. What is the study treatment? The study is testing a new kind of treatment called gene therapy. The drug being studied is called ST-503. It works by reducing certain pain signals in the body. Specifically, it targets something in the nerves called Nav1.7 sodium channels, which are linked to pain. This is a first human study and the study drug is not FDA approved.