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 : S - 53482 and S - 23121 are N - phenylimide herbicides and produced embryolethality , teratogenicity ( mainly ventricular septal defects and wavy ribs ) , and growth retardation in rats in conventional oral developmental toxicity studies .
### Output Text:
BACKGROUND : <span class="chemical">S - 53482</span> and <span class="chemical">S - 23121</span> are N - phenylimide herbicides and produced embryolethality , teratogenicity ( mainly ventricular septal defects and wavy ribs ) , and growth retardation in rats in conventional oral developmental toxicity studies . 