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: HBeAg was positive in 40 % and anti - HBe in 60 % of the patients .
### Output Text:
<span class="chemical">HBeAg</span> was positive in 40 % and anti - HBe in 60 % of the patients . 