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: Increased vulnerability to 6 - hydroxydopamine lesion and reduced development of dyskinesias in mice lacking CB1 cannabinoid receptors .
### Output Text:
Increased vulnerability to <span class="chemical">6 - hydroxydopamine</span> lesion and reduced development of dyskinesias in mice lacking CB1 cannabinoid receptors . 