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: Following unilateral 6 - OHDA induced SN lesion , a transient period of contralateral rotation has been reported to precede the predominant ipsilateral circling .
### Output Text:
Following unilateral <span class="chemical">6 - OHDA</span> induced SN lesion , a transient period of contralateral rotation has been reported to precede the predominant ipsilateral circling . 