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: Two courses of 21 days of 5 mg norethisterone daily caused reversion to normal in all 57 cases of cystic hyperplasia and 6 of the 8 cases of atypical hyperplasia .
### Output Text:
Two courses of 21 days of 5 mg <span class="chemical">norethisterone</span> daily caused reversion to normal in all 57 cases of cystic hyperplasia and 6 of the 8 cases of atypical hyperplasia . 