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: There is no difference in breast - cancer frequency between the placebo ( 22 cases ) and tamoxifen ( 19 ) arms .
### Output Text:
There is no difference in breast - cancer frequency between the placebo ( 22 cases ) and <span class="chemical">tamoxifen</span> ( 19 ) arms . 