Yuri Milner (as an individual and not via DST-Global), along with SV Angel's Start Fund, invested in the individual companies that participated in YC -- not in YC directly.

Though Sequoia Capital is a LP in YC.

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http://news.ycombinator.com/item?id=2156560

http://techcrunch.com/2011/01/28/yuri-milner-sv-angel-offer-...