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: METHODS : Patients receiving an OLTX only from June 1985 through December of 1994 who survived 6 months postoperatively were studied ( n = 834 ) .
### Output Text:
METHODS : Patients receiving an OLTX only from June 1985 through December of 1994 who survived 6 months postoperatively were studied ( n = 834 ) . 