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: It differs from erythromycin only in the methylation of the hydroxyl group at position 6 .
### Output Text:
It differs from <span class="chemical">erythromycin</span> only in the methylation of the hydroxyl group at position 6 . 