A potential human DNA helicase, RECQL, was partially purified from HeLa cells, and a cDNA encoding this protein was subsequently cloned from a HeLa library. The RECQL cDNA contains a protein coding region of 1977 base pairs, and encodes a 659-amino-acid polypeptide with a predicted M(r) 72,000. This predicted protein sequence contains several domains that have extensive sequence identity with similar domains of the RecQ protein from Escherichia coli that has been shown to be an ATP-dependent DNA helicase. Overall amino acid sequence identity between the two proteins is 32%; overall sequence similarity is 57%. The similarities between the two sequences are particularly high in the seven consecutive domains characteristic of DNA and RNA helicases. Expression of the RECQL cDNA in reticulocyte lysates and in transiently transfected cells confirms the M(r) 72,000 of the protein; this protein reacted with antibodies raised against synthetic peptides comprising both the predicted amino- and carboxyl-terminal sequences. These antibodies demonstrated that RECQL is located predominantly in the nucleus of human fibroblasts. Based on its sequence similarity to the E. coli RecQ protein, it is possible that the putative human helicase RECQL may play a role in the repair of DNA that is damaged by ultraviolet light or other mutagens.