DNA polymerase theta (POLQ) performs strand displacement DNA synthesis during microhomology-mediated end joining (MMEJ) (Kent et al. 2015), which is expected to result in the formation of displaced 5'-ssDNA flaps. FEN1, a 5'-flap endonuclease, is a necessary participant of MMEJ (Liang et al. 2005, Sharma et al. 2015). By analogy with base excision repair (Klungland and Lindahl 1997, Liu et al. 2005), FEN1 is thought to cleave the 5'-flaps generated by POLQ-mediated DNA strand displacement synthesis during MMEJ, thus enabling the subsequent ligation step.