Structural domains comprising this superfamily share the structure of the C-terminal domain of the he human cleavage and polyadenylation factor I subunit Clp1, highly conserved in eukaryotic organisms. Both human Clp1 (hsClp1) and Saccharomyces cerevisiae Clp1 (scClp1) were identified as components of the pre-mRNA 3'end-processing machinery. Whereas hsClp1 was shown to also be part of the tRNA endonuclease complex, scClp1 exclusively contributes to mRNA maturation and appears to be enzymatically inactive.

Clp1 is a three domain protein and this entry is focused on the C-terminal domain (CtD) whose functional relevance remains elusive. However, although this CtD does not contribute directly to the active site, it positions the lid module and its truncation most likely indirectly destabilizes the PNK (middle) domain PMID:24813946 PMID:24508575.