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: Twenty - three patients ( 38 % ) and 15 ( 26 % ) , in the combination and zidovudine groups , respectively , discontinued therapy ( p = 0 . 15 ) .
### Output Text:
Twenty - three patients ( 38 % ) and 15 ( 26 % ) , in the combination and <span class="chemical">zidovudine</span> groups , respectively , discontinued therapy ( p = 0 . 15 ) . 