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: There was a significant reduction of thymidine incorporation into both erythroid and endothelial cells in cultures pre - treated with IGF - IL - 3 and EPO .
### Output Text:
There was a significant reduction of <span class="chemical">thymidine</span> incorporation into both erythroid and endothelial cells in cultures pre - treated with IGF - IL - 3 and EPO . 