Component of the cleavage factor Im (CFIm) complex that plays a key role in pre-mRNA 3'-processing. Involved in association with CPSF6 or CPSF7 in pre-MRNA 3'-end poly(A) site cleavage and poly(A) addition. NUDT21/CPSF5 binds to cleavage and polyadenylation RNA substrates. The homodimer mediates simultaneous sequence-specific recognition of two 5'-UGUA-3' elements within the pre-mRNA. Binds to, but does not hydrolyze mono- and di-adenosine nucleotides. May have a role in mRNA export. Homodimer. Component of the cleavage factor Im (CFIm) complex (By similarity). Forms a complex with cleavage and polyadenylation specificity factor (CPSF) subunits FIPS5 (PubMed:16282318, PubMed:18479511). In punctate subnuclear structures localized adjacent to nuclear speckles, called paraspeckles. Belongs to the Nudix hydrolase family. CPSF5 subfamily.