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: It is generally assumed that DNA elongation is stopped by the insertion of AZT into the chain in place of thymidine thus preventing the phosphate hydroxyl linkages and therefore suppresses hemopoietic progenitor cell proliferation in an early stage of differentiation .
### Output Text:
It is generally assumed that DNA elongation is stopped by the insertion of <span class="chemical">AZT</span> into the chain in place of <span class="chemical">thymidine</span> thus preventing the <span class="chemical">phosphate</span> hydroxyl linkages and therefore suppresses hemopoietic progenitor cell proliferation in an early stage of differentiation . 