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The National Library of Medicine is making every effort to ensure that the information available on is accessible to all. Some elements in the PDF/A documents posted on the website may not be fully accessible to persons with disabilities, even with assistive technology. If you are a person with a disability or use assistive technology and encounter problems when using our site and its contents, please let us know. We will work with you to provide the information in an alternate format. Contact Customer Service at NLM Customer Support or phone us at (888) FIND-NLM (346-3656) or (301) 594-5983 and provide the address (URL) of the page on which you need assistance, along with a description of the problem.

See the NLM Accessibility page for more information.

This page last reviewed in November 2020