The Component of the Cleavage and Polyadenylation Specificity Factor (CPSF) complex plays an important role in the 3'-end formation of pre-mRNA. This complex recognizes the AAUAAA signal sequence and interacts with poly(A) polymerase to process and add to the poly(A) tail. FIP1L1 (FIP1-like 1), also known as Pre-mRNA 3'-end-processing factor FIP1, FIP1 (Factor interacting with PAP) and RHE (Rearranged in hypereosinophilia), is a 594 amino acid nuclear protein that is a component of the CPSF complex. Within the complex, FIP1L1 contributes to the poly(A) recognition and stimulates poly(A) addition. Fusion of the genes encoding FIP1L1 and PDGFRA due to an interstitial deletion on chromosome 4q12 is the cause of hypereosinophilia syndrome, a rare blood disorder characterized by continuous overproduction of eosinophils in the bone marrow that leads to tissue infiltration and organ damage. There are three isoforms of FIP1L1 that are produced as a result of alternative splicing events.