ATP-dependent 3'-5' DNA helicase, unwinds 3'-overhangs, 3'- flaps, and splayed-arm DNA substrates but not 5'-overhangs or 5'-flap substrates. Requires ATP hydrolysis for activity; the ATPase activity is DNA-dependent and requires a minimum of 4 single-stranded nucleotides (nt) with 6-10 nt providing all necessary interactions for full processive unwinding. The ATPase prefers ATP over CTP or GTP, is almost inactive with TTP. ATP + H2O = ADP + H(+) + phosphate ATPase activity has a small preference for Mn(2+) over Mg(2+) or Ca(2+). Co(2+) and Zn(2+) are inactive. Monomer. Removal of the N-terminus decreases solubility and/or structural integrity of the protein. Belongs to the helicase family. RAD25/XPB subfamily.