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: Although this new combination appears to be effective as a prophylactic regimen for acute GVHD , the incidence of TMA and SOS is considerably higher than seen with other regimens .
### Output Text:
Although this new combination appears to be effective as a prophylactic regimen for acute GVHD , the incidence of TMA and SOS is considerably higher than seen with other regimens . 