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: A lesion induced by 6 - OHDA produced more severe motor deterioration in CB1 KO mice accompanied by more loss of DA neurons and increased PENK gene expression in the CPu .
### Output Text:
A lesion induced by <span class="chemical">6 - OHDA</span> produced more severe motor deterioration in CB1 KO mice accompanied by more loss of <span class="chemical">DA</span> neurons and increased <span class="chemical">PENK</span> gene expression in the CPu . 