- The 3'-->5' exonuclease of DNA polymerase delta can substitute for the 5' flap endonuclease Rad27/Fen1 in processing Okazaki fragments and preventing genome instability. Jin, Y.H., Obert, R., Burgers, P.M., Kunkel, T.A., Resnick, M.A., Gordenin, D.A. Proc. Natl. Acad. Sci. U.S.A. (2001)