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: Guillain - Barr syndrome was the commonest identifiable cause ( 15 . 6 % ) , accounting for half of the cases with motor neuropathy .
### Output Text:
Guillain - Barr syndrome was the commonest identifiable cause ( 15 . 6 % ) , accounting for half of the cases with motor neuropathy . 