We showed that DNA-dependent ATPase Ql (DNA helicase Ql) from xeroderma pignientosiim complementation group C (XP-C) cells elutes from FPLC Mono Q column at higher concentrations of KC1 than that from other human cells (35). We purified DNA helicase Ql from XP-C and HeLa cells. The purified fractions of both cells contained a major polypeptide with a molecular mass of 73 kDa and had the same enzymatic properties, including salt- and temperature-sensitivity. Characterization using an anti-DNA helicase Ql antibody indicated that this enzyme localized in the nuclei and was not modified by incorporating phosphate groups through phosphorylation and ADP-ribosylation. No interactions of DNA helicase Ql with other proteins were indicated by immunoprecipitation of the helicase from crude extracts. No difference was observed in XP-C cells in intracellular localization of DNA helicase Ql, phosphorylation, and the interaction with other proteins as compared to HeLa cells.