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: UNASSIGNED : Epidemiological studies of 1 , 3 - butadiene have suggest that exposures to humans are associated with chronic myeloid leukemia ( CML ) .
### Output Text:
UNASSIGNED : Epidemiological studies of <span class="chemical">1 , 3 - butadiene</span> have suggest that exposures to humans are associated with chronic myeloid leukemia ( CML ) . 