Oh yeah, it’s here: Chance and Providence (reprised) - #139 by Rumraket
Here’s what I wrote there:
Simply looking up the Prp8 protein on wikipedia will tell you there are human Prp8 mutants living among us, and some of these apparently negatively affect vision, leading some of them eventually go blind in adulthood. That’s strongly deleterious, which would explain why such mutants haven’t fixed in the population. But they’re clearly not lethal, since there are people running around with them.