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: These observations indicate that although the gamma - carboxyglutamate residues of matrix Gla protein are apparently required for its function as a calcification inhibitor , they are not required for its accumulation at calcification sites .
### Output Text:
These observations indicate that although the <span class="chemical">gamma - carboxyglutamate</span> residues of matrix Gla protein are apparently required for its function as a calcification inhibitor , they are not required for its accumulation at calcification sites . 