According to the literature, members of the structural domains which comprise this superfamily correspond to the structure of the C-terminal domain in the PB2 subunit of the trimeric influenza virus polymerase. This is an RNA polymerase involved in the transcription and replication of its segmented RNA genome. The C-terminal domain of PB2 forms a stable complex with human importin alpha5 in vitro which, suggests that this domain directs nuclear import. This is further backed by the presence of a previously missed bipartite nuclear localization sequence (NLS) within this domain PMID:17310249