An official website of the United States government
Here's how you know
Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
. Author manuscript; available in PMC: 2012 Aug 1.
Published in final edited form as: Cold Spring Harb Perspect Biol. 2012 Feb 1;4(2):10.1101/cshperspect.a011593 a011593. doi: 10.1101/cshperspect.a011593