a component of the cleavage and polyadenylation specificity factor (CPSF) complex that plays a key role in pre- mRNA 3'-end formation, recognizing the AAUAAA signal sequence and interacting with poly(A) polymerase and other factors to bring about cleavage and poly(A) addition. FIP1L1 contributes to poly(A) site recognition and stimulates poly(A) addition. Binds to U-rich RNA sequence elements surrounding the poly(A) site. May act to tether poly(A) polymerase to the CPSF complex. A fusion of FIP1L1 and PDGFRA (FIP1L1-PDGFRA) is the cause of some cases of hypereosinophilic syndrome (HES), a disorder characterized by overproduction of eosinophils in the bone marrow, eosinophilia, tissue infiltration and organ damage. Four alternatively spliced isoforms have been described. Note: This description may include information from UniProtKB.