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: During a 9 - year period , we retrospectively collected 27 neurological events ( 11 % ) in as many patients , from 253 children enrolled in the ALL front - line protocol .
### Output Text:
During a 9 - year period , we retrospectively collected 27 neurological events ( 11 % ) in as many patients , from 253 children enrolled in the ALL front - line protocol . 