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: Schizophrenia has been initially associated with dysfunction in dopamine neurotransmission .
### Output Text:
Schizophrenia has been initially associated with dysfunction in <span class="chemical">dopamine</span> neurotransmission . 