FIP1 was originally identified as Rhe (rearranged in hypereosinophilia) and part of a fusion protein encoded by a genetic rearrangement between the N-terminal region of FIP1 (Rhe) and the C-terminal region of the platelet-derived growth factor receptor alpha (PDGFRalpha). The fusion protein exhibits aberrant tyrosine kinase activity and is associated with hypereosinophilic syndrome (HES), a rare hematologic disorder characterized by an overproduction of eosinophils that results in organ damage. FIP1 was also identified as FIP1-like 1 in a database search for a human homolog of the yeast Fip1p (factor interacting with PAP), a subunit of the yeast 3’-end processing complex, CPF (cleavage and polyadenylation factor). Human FIP1 was similarly found to interact with PAP and CPSF160 as part of the human CPSF (cleavage and polyadenylation specificity factor) complex. In association with CPSF160, FIP1 is able to recognize U-rich elements on pre-mRNA and stimulate PAP activity.