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: Twelve patients with liver disease related to methyldopa were seen between 1967 and 1977 .
### Output Text:
Twelve patients with liver disease related to <span class="chemical">methyldopa</span> were seen between 1967 and 1977 . 