P-LAP is identical with cystine aminopeptidase as oxytocinase. We previously located on the placental leucine aminopeptidase (P-LAP) gene the footprint site with the high promoter activity (FP3: -214 to -183) and found a possible interaction between it and AP-2 in choriocarcinoma cells. Here, we investigated FP3 in detail and identified the elements responsible for the high basal rate of transcription. Electrophoretic mobility shift assays demonstrated that FP3 would interact with Ikaros as well as AP-2. Further analysis using antibody against Ikaros confirmed that Ikaros was indeed present and bound to FP3. In addition, AP-2alpha and AP-2,gamma antibodies supershifted the second complex at FP3. Functionally, mutations that eliminate AP-2 binding reduced promoter activity significantly, while those that eliminate Ikaros binding reduced promoter activity insignificantly. Double mutations of AP-2 and Ikaros decreased promoter activity progressively. We conclude that AP-2 is the main activator and Ikaros functions cooperatively with it for maximal expression of the human P-LAP gene. (C) 2002 Elsevier Science (USA).