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: Testosterone ( 1 mg . kg ( - 1 ) . d ( - 1 ) , sc ) , the androgen receptor antagonist flutamide ( 10 mg . kg ( - 1 ) . d ( - 1 ) , ip ) , the estrogen receptor antagonist tamoxifen ( 1 mg . kg ( - 1 ) . d ( - 1 ) , ip ) or the aromatase inhibitor letrozole ( 4 mg . kg ( - 1 ) . d ( - 1 ) , ip ) were administered for 6 d after the first injection of STZ .
### Output Text:
<span class="chemical">Testosterone</span> ( 1 mg . kg ( - 1 ) . d ( - 1 ) , sc ) , the <span class="chemical">androgen</span> receptor antagonist <span class="chemical">flutamide</span> ( 10 mg . kg ( - 1 ) . d ( - 1 ) , ip ) , the <span class="chemical">estrogen</span> receptor antagonist <span class="chemical">tamoxifen</span> ( 1 mg . kg ( - 1 ) . d ( - 1 ) , ip ) or the aromatase inhibitor <span class="chemical">letrozole</span> ( 4 mg . kg ( - 1 ) . d ( - 1 ) , ip ) were administered for 6 d after the first injection of <span class="chemical">STZ</span> . 