Pituitary tumor transforming gene (PTTG) is a multifunctional oncogene implicated in the pathogenesis of pituitary and thyroid tumours. In breast cancer, PTTG expression is a prognostic marker for lymph node invasion and tumour recurrence. PTTG expression is regulated in vitro and in vivo by oestrogen treatment. PTTG binding factor (PBF) is also a transforming gene, and was isolated due to its functional interaction with PTTG. In the current study, we examined PBF regulation by oestrogen and progesterone in breast cancer MCF-7 cells. PBF mRNA expression was induced maximally at 48 h by 20 nM diethylstilbestrol (2.1±0.1-fold, P<0.001, N=6), 10 nM progesterone (1.8±0.04-fold, P<0.05, N=6) and by diethylstilbestrol plus progesterone (10 nM of each, 1.9±0.3-fold, P<0.05, N=6). PBF protein expression levels were also significantly up-regulated by diethylstilbestrol (2.4±0.6-fold, P<0.05, N=3), and PTTG protein expression levels were similarly up-regulated by diethylstilbestrol (2.8±0.7-fold, P<0.05, N=3) and by diethylstilbestrol plus progesterone (2.1±1.1-fold, P<0.05, N=3). Luciferase assays confirmed that the first 1 Kb of the human PBF promoter conveys PBFs transcriptional response to oestrogen. In accord with this, we identified multiple putative oestrogen response elements (EREs) in the human PBF promoter. Critically, we found that the region −399 to −291 relative to the transcriptional start site contains variable repeats of an 18 bp sequence housing a putative ERE half-site (gcccctcGGTCAcgcctc). Sequencing the PBF promoter from 69 normal DNA samples revealed that individuals may be homozygous or heterozygous for between 1 and 6 repeats of the ERE, with overall allele frequencies of 1.4% (1 ERE), 85.5% (3 EREs), 0.8% (4 EREs), 8.7% (5 EREs) and 3.6% (6 EREs). Given that PBF and PTTG are potent transforming genes, we propose that oestrogen treatment in post-menopausal women up-regulates PTTG and PBF expression, eliciting mechanisms of cell transformation. Further, the number of ERE half sites in the PBF promoter may significantly alter the response to oestrogen treatment in different patients.