We describe the cloning and characterization of a new human Xq13 gene (XH2), extending over a 220 kb genomlc stretch between MNK and DXS56. The gene, which undergoes X-inactivatlon, contains a 4 kb open reading frame and encodes a putative NTP-blndlng nuclear protein homologous to several members of the helicase II superfamily. The murine homologue maps to the syntenlc genetic interval, between Pgk1 and Xlst. In situ hybridization studies In mouse reveal precocious, widespread expression of the murine homologue of XH2 at early stages of embryogenesis, and more restricted expression during late developmental stages and at birth. XH2 Is a new member of an expanding family of proven and putative helicases, sharing six conserved, colllnear domains. In particular, the XH2 protein shows homology with yeast RAD54. Type II helicases have been Implicated in nucleotide excision repair and the initiation of transcription. This new gene, represents a potential candidate for several genetic disorders mapped to human Xq13.