If you see an error message when trying to load an NCBI web page, there are several reasons this could be happening. Access problems fall into three general categories:  

•  Server errors
•  DNS problems
•  Access restrictions

Server Errors 

Server errors (such as 500 Internal Server Error) may happen on NLM servers. They can also happen within your web browser. To resolve:

1.  Close your browser application completely and reopen.
2. Clear your browser’s cache and any NLM or NIH browser cookies. For instructions within your browser:
   • Check Preferences or Settings. Look at the areas under Privacy or Security.
3.  Try your request again later as servers become more available.

If the problem persists, write to Help Desk to report the issue.

DNS Problems

If you get This webpage is not available error message, check your link first. Try accessing the page that you need from another NCBI page. A local Domain Name System (DNS) server problem can cause the same error message. To resolve DNS problems, contact your system administrator or Internet Service Provider (ISP).

If the problem persists, write to Help Desk to report the issue.