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: Decrease of fetal length was the only signs of the DFU developmental toxicity observed in pups exposed to the highest compound dose .
### Output Text:
Decrease of fetal length was the only signs of the <span class="chemical">DFU</span> developmental toxicity observed in pups exposed to the highest compound dose . 