Gene fusions between the mouse mammary tunor virus long terminal repeat and the E. coli lacZ gene have been shown to exhibit hormone dependent expression of β-galactosidase activity. These constructions were used in transient expression experiments to assess the effects of specific modifications introduced into the region upstream of the transcription initiation site. 5′ deletions denonstrate that sequences sufficient for wild-type promoter function are contained downstream of residue -64 relative to the initiation site. Other deletions define a region of approximately 80 base pairs between -220 arid -140 which contains sequences essential for hormonal control. Between this control region and the promoter lie sequences dispensable for both functions.