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: Central nervous system ( CNS ) complications during treatment of childhood acute lymphoblastic leukemia ( ALL ) remain a challenging clinical problem .
### Output Text:
Central nervous system ( CNS ) complications during treatment of childhood acute lymphoblastic leukemia ( ALL ) remain a challenging clinical problem . 