RecruitingGenetic interventionSafety and Tolerability Study of ST-503 for Refractory Pain Due to Peripheral Neuropathy (Small Fiber Predominant, SFN)This research is being done to study a possible treatment for refractory pain due to small fiber neuropathy (SFN). ST-503 is intended to deliver a modified copy of the gene which will ideally repress Nav1.7 tissue-related pain signals reaching the brain, which should reduce the refractory pain due to small fiber neuropathy (SFN).