In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: They also provide evidence for estradiol - dependent and brain region - specific regulation of PRL - R expression and signaling responses by prolactin .
### Output Text:
They also provide evidence for <span class="chemical">estradiol</span> - dependent and brain region - specific regulation of PRL - R expression and signaling responses by prolactin . 