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: To this end , doxorubicin ( 15 mug / g body wt ) was injected intravenously into gene - targeted mice lacking SGK1 ( sgk1 ( - / - ) ) and their wild - type littermates ( sgk1 ( + / + ) ) .
### Output Text:
To this end , <span class="chemical">doxorubicin</span> ( 15 mug / g body wt ) was injected intravenously into gene - targeted mice lacking SGK1 ( sgk1 ( - / - ) ) and their wild - type littermates ( sgk1 ( + / + ) ) . 