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: An analysis for factor V Leiden and the G20210A mutation in the prothrombin gene was conducted in 217 patients and 763 controls RESULTS : The odds ratio for myocardial infarction among women who used any type of combined oral contraceptive , as compared with nonusers , was 2 . 0 ( 95 percent confidence interval , 1 . 5 to 2 . 8 ) .
### Output Text:
An analysis for factor V Leiden and the G20210A mutation in the prothrombin gene was conducted in 217 patients and 763 controls RESULTS : The odds ratio for myocardial infarction among women who used any type of combined <span class="chemical">oral contraceptive</span> , as compared with nonusers , was 2 . 0 ( 95 percent confidence interval , 1 . 5 to 2 . 8 ) . 