[Beldur]

At the press conference after the 4th game it was mentioned that AlphaGo was not trained on professional games at all.

They used strong amateur player games from public archives.

It was also mentioned that AlphaGo needs millions of games to be trained. Even if they added a few hundred Lee Sedol games, the impact would be very small.

[bainsfather]

You are correct. They used the large number of amateur games from the KGS Go server.

By the way, AlphaGo has a wikipedia page: https://en.wikipedia.org/wiki/AlphaGo

And the Nature paper is: http://www.nature.com/nature/journal/v529/n7587/full/nature1... (paywalled, but scihub can get it for you). The paper is fairly readable, and worth having a look at if you are interested in this topic.