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: Of those positive for p24 antigen at the commencement of the study 67 % showed a positive response , and this was most likely in those with CD4 + lymphocyte subset counts above 100 mm3 .
### Output Text:
Of those positive for p24 antigen at the commencement of the study 67 % showed a positive response , and this was most likely in those with CD4 + lymphocyte subset counts above 100 mm3 . 