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: Our study group consisted of 74 HIV - positive homosexual men belonging to groups II B , III and IV C2 from the Centers for Disease Control ( CDC ) classification of HIV disease .
### Output Text:
Our study group consisted of 74 HIV - positive homosexual men belonging to groups II B , III and IV C2 from the Centers for Disease Control ( CDC ) classification of HIV disease . 