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 : In man , differences in angiotensin - converting enzyme ( ACE ) levels , related to ACE ( I / D ) genotype , are associated with renal prognosis .
### Output Text:
BACKGROUND : In man , differences in <span class="chemical">angiotensin</span> - converting enzyme ( ACE ) levels , related to ACE ( I / D ) genotype , are associated with renal prognosis . 