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: Hannover Wistar rats were dosed with 1 . 25 mg / kg DOX weekly for 8 weeks followed by a 4 week off - dosing " recovery " period .
### Output Text:
Hannover Wistar rats were dosed with 1 . 25 mg / kg <span class="chemical">DOX</span> weekly for 8 weeks followed by a 4 week off - dosing " recovery " period . 