This HIV-1 event was inferred from the corresponding human RNA Pol II transcription event. FCP1 dephosphorylates RNAP II in ternary elongation complexes as well as in solution and, therefore, is thought to function in the recycling of RNAP II during the transcription cycle. Biochemical experiments suggest that human FCP1 targets CTDs that are phosphorylated at serine 2 (CTD-serine 2) and/or CTD-serine 5. It is also observed to stimulate elongation independent of its catalytic activity. Dephosphorylation of Ser2 - phosphorylated Pol II results in hypophosphorylated form that disengages capping enzymes (CE).