Summary of mechanisms for domain gains. This figure shows potential mechanisms leading to domain gains and the signals that can be used to detect the causative mechanism. Domain gain by retroposition is illustrated as an example where the domain is transcribed together with the upstream long interspersed nuclear element (LINE), but other means of retroposition are also possible . The list of possible mechanisms is not exhaustive and other scenarios can occur, such as, for example, exonization of previously non-coding sequence or gain of a viral or transposon domain during retroelement replication. IR, illegitimate recombination; NAHR, non-allelic homologous recombination.