DeNA, a big mobile gaming company in Japan, is in talks to acquire San Francisco-based iOS game developer ngmoco for as much as $400 million, according to sources.

One of the most popular social game developers for iPad, iPhone, and iPod Touch, ngmoco has released 24 games to date, including titles in the “We” series like We City and We Farm, Godfinger, Topple, and Star Defense.

ngmoco has raised over $40 million from prominent Silicon Valley investors like Institutional Venture Partners (IVP), Kleiner Perkins, Norwest Venture Partners, and Maples Investments. Curiously enough, the startup shares two of these, IVP and Kleiner Perkins, with another successful social gaming company: Zynga.

Zynga CEO Mark Pincus, however, has made it stunningly clear that he’s not interested in an exit right now. And why should he? His startup has raised hundreds of million dollars, continues to acquire excellent talent left and right, and shows only signs of good health.

While ngmoco hasn’t raised hundreds of millions of dollars, the startup recently picked up an estimated $3-5 million from Google Ventures, Google’s venture capital arm. Expectedly, ngmoco now has plans to develop games for Android.

The DeNA acquisiton would not be unprecedented. In September, DeNA acquired Gameview and invested in Astro Ape; both are Silicon Valley startups developing social mobile games for Android and iPhone.

Mobile gaming has already gone a lot further in Japan, where the market is worth $1 billion in revenue for DeNA this year alone.