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: Neutropenia was the principal toxic effect , and the overall frequency of grade 4 neutropenia after the first treatment of CCNU / CTX was 30 % ( 95 % confidence interval , 19 - 43 % ) .
### Output Text:
Neutropenia was the principal toxic effect , and the overall frequency of grade 4 neutropenia after the first treatment of <span class="chemical">CCNU</span> / <span class="chemical">CTX</span> was 30 % ( 95 % confidence interval , 19 - 43 % ) . 