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: Estradiol is known to influence expression of the long form of prolactin receptors ( PRL - R ) and components of prolactin ' s signaling pathway .
### Output Text:
<span class="chemical">Estradiol</span> is known to influence expression of the long form of prolactin receptors ( PRL - R ) and components of prolactin ' s signaling pathway . 