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: PARP - 1 + / + and PARP - 1 - / - mice received a single injection of DOX ( 25 mg / kg i . p ) .
### Output Text:
PARP - 1 + / + and PARP - 1 - / - mice received a single injection of <span class="chemical">DOX</span> ( 25 mg / kg i . p ) . 