Once again, Eli Lilly and Co. has signed a billion-dollar deal, this time with Boston-based Ascidian Therapeutics Inc., a company that is barely four years old and one that is focused on treating human diseases by rewriting RNA. “Our technology, we call it RNA exon editing,” said Ascidian Chief Scientific Officer Robert Bell. “It edits RNA, not DNA … but it does so at the kilobase scale.”