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: Endothelial cell culture supernatants separated by ultrafiltration and ultracentrifugation from cells treated with EPO and IL - 3 significantly reduced thymidine incorporation into erythroid cells as compared to identical fractions obtained from the media of cells cultured with EPO alone .
### Output Text:
Endothelial cell culture supernatants separated by ultrafiltration and ultracentrifugation from cells treated with EPO and IL - 3 significantly reduced <span class="chemical">thymidine</span> incorporation into erythroid cells as compared to identical fractions obtained from the media of cells cultured with EPO alone . 