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: With this low frequency of extrapyramidal symptoms , it was thought the frequency of neuroleptic malignant syndrome might also be lowered .
### Output Text:
With this low frequency of extrapyramidal symptoms , it was thought the frequency of neuroleptic malignant syndrome might also be lowered . 