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: For patients with prior zidovudine therapy , the mean change in CD4 cells in the combination and zidovudine groups was 63 . 7 cells / mm3 and 4 . 9 cells / mm3 at week 8 and 6 . 8 cells / mm3 and - 45 . 1 cells / mm3 at week 16 , respectively .
### Output Text:
For patients with prior <span class="chemical">zidovudine</span> therapy , the mean change in CD4 cells in the combination and <span class="chemical">zidovudine</span> groups was 63 . 7 cells / mm3 and 4 . 9 cells / mm3 at week 8 and 6 . 8 cells / mm3 and - 45 . 1 cells / mm3 at week 16 , respectively . 