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: These data suggest that exposure of animals to BCNU during pregnancy leads to delayed maturation of offspring cerebellum and melatonin protects the cerebellum against the effects of BCNU .
### Output Text:
These data suggest that exposure of animals to <span class="chemical">BCNU</span> during pregnancy leads to delayed maturation of offspring cerebellum and <span class="chemical">melatonin</span> protects the cerebellum against the effects of <span class="chemical">BCNU</span> . 