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# Guide on Slowly Progressive, Low-Prevalence Rare Diseases
- URL: https://www.fdaweb.com/guide-on-slowly-progressive-low-prevalence-rare-diseases/
- Published: 2020-03-16T12:00:00.000Z
- Updated: 2026-09-14T16:07:26.000Z
- Author: David McFarland
- Tags: FDA Policy/General, #legacy-id-D5146329

*Federal Register* notice: FDA has made available a final guidance entitled “Slowly Progressive, Low-Prevalence Rare Diseases With Substrate Deposition That Results From Single Enzyme Defects: Providing Evidence of Effectiveness for Replacement or Corrective Therapies.” The document discusses the evidence necessary to demonstrate the effectiveness of investigational new drugs or new drug uses intended for such indications. It applies only to those low-prevalence rare diseases with a well-characterized pathophysiology and in which changes in substrate deposition can be readily measured in relevant tissue or tissues. To view the notice, click [here](https://www.govinfo.gov/content/pkg/FR-2020-03-16/pdf/2020-05335.pdf?ref=fdaweb.com).