HeLa nuclear extract was prepared as previously described [13 (link)]. Extracts were incubated with Protein A-Dynal beads coupled with polyclonal antibody against human RECQ1 (A300-450A, Bethyl Lab), FEN-1 (A300-255A, Bethyl Lab) or normal rabbit IgG (Vector Labs) at 4°C for 90 min, and the immunecomplexes were eluted with 2x SDS-sample buffer following three washes with lysis buffer. Where indicated, nuclear extract was pre-incubated with benzonase (Sigma, 50 U/ml, 2 h at 4°C), a general endonuclease for DNA and RNA. Proteins were resolved by 8-16% SDS-PAGE, transferred onto PVDF membrane and subjected to Western detection of RECQ1 (1:750, sc-25547, Santa Cruz Biotech), FEN-1 (1:1000, Bethyl Lab).