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: Mean follow - up on SRL therapy was 20 + / - 12 ( 6 to 43 ) months .
### Output Text:
Mean follow - up on <span class="chemical">SRL</span> therapy was 20 + / - 12 ( 6 to 43 ) months . 