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: A calcineurin inhibitor combined with methotrexate is the standard prophylaxis for graft - versus - host disease ( GVHD ) after allogeneic hematopoietic stem cell transplantation ( HSCT ) .
### Output Text:
A calcineurin inhibitor combined with <span class="chemical">methotrexate</span> is the standard prophylaxis for graft - versus - host disease ( GVHD ) after allogeneic hematopoietic stem cell transplantation ( HSCT ) . 