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: BACKGROUND : Current animal models of obsessive - compulsive disorder ( OCD ) typically involve acute , drug - induced symptom provocation or a genetic association with stereotypies or anxiety .
### Output Text:
BACKGROUND : Current animal models of obsessive - compulsive disorder ( OCD ) typically involve acute , drug - induced symptom provocation or a genetic association with stereotypies or anxiety . 