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: Serum concentrations of prolactin ( PRL ) and testosterone ( T ) were measured .
### Output Text:
Serum concentrations of prolactin ( <span class="chemical">PRL</span> ) and <span class="chemical">testosterone</span> ( <span class="chemical">T</span> ) were measured . 