The human X-linked steroid sulfatase gene (STS) was among the first genes shown to escape X inactivation. At least fourteen genes regulated in this fashion have now been recognized. They are dispersed into several regions of the X chromosome and may be controlled in a locus specific manner. Studies of the promoters of these genes could provide insights into the mechanism of X inactivation, however little information of this nature is currently available. For this reason we examined 5' flanking sequences of the human STS gene for promoter function. Four transcription start sites scattered over a 50bp region were identified. Functional domains of this TATA-less and GC poor promoter were identified by study of a series of terminal and internal deletions. A putative promoter sequence was identified which by itself exhibits little or no basal activity. However when combined with upstream regulatory elements, this segment showed weak but reproducible activity in a CAT (chloramphenicol acetyltransferase) reporter assay. Several regulatory domains acting as enhancers and repressors were subsequently identified. The relationship of this 5' sequence to the ability of the STS gene to escape X-inactivation is discussed.