Cell lines
HEK 293FT cells were a kind gift from Dr G. Stewart while HeLa and U2OS cells were a generous gift from Dr F. Esashi. These cell lines were cultured in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and standard antibiotics. U2OS cells stably expressing GFP-CtIP and U2OS cells harbouring the HR reporter DR-GFP were a generous gift from Prof. S.P. Jackson and were maintained in media supplemented with 500 μg ml−1 G-418. The ER-AsiSI U2OS cell line 47, a kind gift from Prof. G. Legube, was maintained in DMEM media without phenol red supplemented with 10% dialysed FBS (Life Technologies) and 1 μg ml−1 Puromycin. Cell lines stably expressing FLAG-HA EXD2 WT or D108A/E110A fusion proteins were generated by transfection of U2OS cells with these plasmid constructs followed by clonal selection of cells grown in media containing 500 μg ml−1 G418 (Life Technologies). All cell lines have been verified mycoplasma free by PCR based test (Takara).