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: The mean doses of MZ and FL were 4 . 3 + / - 1 . 9 mg and 0 . 28 + / - 0 . 2 mg , respectively .
### Output Text:
The mean doses of <span class="chemical">MZ</span> and <span class="chemical">FL</span> were 4 . 3 + / - 1 . 9 mg and 0 . 28 + / - 0 . 2 mg , respectively . 