Correct term for initially neutral mutations that subsequently cannot be reversed Can anyone tell me what the correct name is for a neutral mutation to a protein that occurs when that mutation becomes "locked in" due to a subsequent mutation? For example: mutation A is neutral initially and can be reversed with no deleterious effects, but after mutation B, the reversal of mutation A would now become deleterious. What is the correct terminology for a mutation of A's type?

Mutation A is a _suppressor_ of Mutation B. The Wikipedia article on the topic (< mostly describes situations where Mutation A occurs after B and reverses B's deleterious effects, but the principle is the same.