1. Hormones such as estrogen (or a mimic such as BPA) work by binding to specific cells.

2. The hormone can enter the nucleus of the cell and bind to the DNA.

3. This binding may activate or inactivate genes in the DNA.

4. In turn, this affects the production of proteins encoded by the gene.

5. The protein can have various effects on the cell's function.

Thus BPA is a endocrine disruptor, upsetting the delicate balance of cellular processes that are regulated by hormones.