The results presented here demonstrate that p53 upregulates estrogen receptor-alpha (ER alpha) expression in the human breast cancer cell line MCF-7. Two approaches were used to alter the activity of p53 in the cells. In the first approach, stable transfectants expressing an antisense p53 were established. In the stable clones, expression of antisense p53 resulted in a decrease in the expression of ER alpha protein. In the second approach, MCF-7 cells were transiently transfected with wild-type p53. Overexpression of p53 increased the amount of ER alpha. To determine whether the effects of p53 on the expression of ER alpha were due to changes in transcription, deletion mutants of the ER alpha promoter were used. This experimental approach demonstrated that p53 up-regulates ER alpha gene expression by increasing transcription of the gene through elements located upstream of promoter A. Transfection assays using p53 mutants further demonstrated that the p53-induced increase in ER alpha gene transcription was not dependent on the ability of p53 to bind to DNA but on its ability to interact with other proteins.