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: SRL was started at a mean of 78 + / - 42 ( 15 to 163 ) months after transplantation .
### Output Text:
<span class="chemical">SRL</span> was started at a mean of 78 + / - 42 ( 15 to 163 ) months after transplantation . 