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: Both ATP and SNP should act on the pacemaker tissue of the heart .
### Output Text:
Both <span class="chemical">ATP</span> and <span class="chemical">SNP</span> should act on the pacemaker tissue of the heart . 