NCBI will transition to HTTPS on September 1, 2016

Starting on September 1st, when you visit NCBI pages, you’ll see a green lock and https:// in the address bar instead of http://. This lets you know that you are really on an NCBI page – that our server identity is confirmed – and that your communication with our server is encrypted and private.
Source: NCBI Announcements - Category: Databases & Libraries Source Type: news