this is a good example of the gap that exists between the plausible mechanism pathway for n=1 diseases like baby KJ vs other ultrarare diseases. even though only a few dozen US patients have ngly1 deficiency, it is too many to qualify for the pathway, so must meet fda standards for gene therapy manufacturing. the article says the company estimates it will cost them $20-25m and 2+ years (that actually sounds like an optimistically low estimate to me). it is precisely at this point that otherwise promising gene therapies hit a brick wall.

https://endpoints.news/grace-science-says-fda-jeopardizes-rare-disease-therapy/?u=cedfead1-db86-4c3f-b8b6-318432f6608d&s=email&c=524130ce-ff1a7f97-9c070832&utm_medium=email&utm_campaign=An%20ultra-rare%20disease%20drugmaker%20faces%20a%20bottleneck%20Insider&utm_content=An%20ultra-rare%20disease%20drugmaker%20faces%20a%20bottleneck%20Insider+CID_fd962452a2e9cb184049f47edd1ea493&utm_source=ENDPOINTS%20emails&utm_term=A%20father%20built%20a%20gene%20therapy%20for%20his%20daughter%20He%20says%20an%20FDA%20request%20is%20the%20bottleneck