One of the multiple factors required for polyadenylation and 3'-end cleavage of pre-mRNAs. May be responsible for the interaction of CSTF with other factors to form a stable complex on the pre-mRNA. Homodimer. Belongs to the CSTF complex (By similarity). Forms a complex with cleavage and polyadenylation specificity factor (CPSF) subunits CSTF64, PABN3, CPSF30, FIPS5 and CPSF100 (PubMed:18479511). N-terminus mediates homodimerization.