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: Subjects were receiving desferrioxamine ( DFO ) chelation treatment with a mean daily dose of 50 - 60 mg / kg , 5 - 6 days a week during the first six years of the study , which was then reduced to 40 - 50 mg / kg for the following eight years .
### Output Text:
Subjects were receiving <span class="chemical">desferrioxamine</span> ( <span class="chemical">DFO</span> ) chelation treatment with a mean daily dose of 50 - 60 mg / kg , 5 - 6 days a week during the first six years of the study , which was then reduced to 40 - 50 mg / kg for the following eight years . 