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: The mean increase in CD4 cells at week 4 was 73 . 8 cells / mm3 and 52 . 4 cells / mm3 for the combination and zidovudine groups , respectively ( p > 0 . 36 ) .
### Output Text:
The mean increase in CD4 cells at week 4 was 73 . 8 cells / mm3 and 52 . 4 cells / mm3 for the combination and <span class="chemical">zidovudine</span> groups , respectively ( p > 0 . 36 ) . 