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: RAPA was discontinued in four patients , because of pneumonia in two , PTLD in one , and oral aphtous ulcers in one .
### Output Text:
<span class="chemical">RAPA</span> was discontinued in four patients , because of pneumonia in two , PTLD in one , and oral aphtous ulcers in one . 