They indeed have partnership with Opera:

http://opera.yandex.ru/

Also with Mozilla:

http://fx.yandex.ru/

and Microsoft:

http://ie.yandex.ru/

:-)

But no, this one is based on Chromium: http://habrahabr.ru/company/yandex/blog/152905/ (in Russian)

EDIT: It says they are using Opera Turbo technology there.

We will see :-) Wondering what they would be using the Opera tech for: http://www.newsweb.no/newsweb/search.do?messageId=312626

Indeed, Holmes. Opera Turbo in the next version of Yandex.Browser.

Also they licensed Opera Store (http://apps.opera.com ) for Yandex Store for Android (http://store.yandex.com ).

(Now that I re-read my comment, I think it can be interpreted in an offensive way. To make things clear, by saying "Holmes" I wanted to compliment you for discovering this information.)