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: A significant and then progressive fall in global LV function was observed at a cumulative doxorubicin dose of 4 mg / kg .
### Output Text:
A significant and then progressive fall in global LV function was observed at a cumulative <span class="chemical">doxorubicin</span> dose of 4 mg / kg . 