Apparently concerned that there are still one or two prominent tech companies that it hasn&'t invested in, venture firm Andreessen Horowitz has purchased more than $80 million of Twitter stock.

It&'s not a direct investment in the company, but rather a stock purchase through the secondary markets where early investors and employees can sell off their shares. Twitter raised a $200 million round late last year, but Andreessen Horowitz did not participate.

All Things Digital broke the news about the investment, and Andreessen Horowitz partner Margit Wennmachers confirmed the story.

With this investment, Andreessen Horowitz can add Twitter to the formidable list of big-name tech companies that it has invested in, which also includes Facebook, Foursquare, Groupon, Skype, and Zynga. (Netscape cofounder Marc Andreessen, one of the firm&'s founding partners, was initially involved with Facebook as an advisor to chief executive Mark Zuckerberg, and Zuckerberg appointed him to Facebook&'s board of directors, but the firm also started buying up Facebook shares on the secondary markets last year.) And that&'s not counting all the slightly-less-famous-but-still-notable startups it has backed.

And the firm has only been in business for less than two years. It has also raised nearly $1 billion to make investments, most recently in a $650 million fund. Earlier this year, the firm said that it models itself on the Creative Arts Agency, a Hollywood talent agency so important that, in VentureWire reporter Deborah Gage&'s words, &''it was hard to do deals without them being involved&''.

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Companies: Andreessen Horowitz, Twitter

People: Marc Andreessen

Companies: Andreessen Horowitz, Twitter

People: Marc Andreessen

Anthony is a senior editor at VentureBeat, as well as its reporter on media, advertising, and social networks. Before joining the site in 2008, Anthony worked at the Hollister Free Lance, where he won awards from the California Newspaper Publishers Association for breaking news coverage and writing. He attended Stanford University and now lives in San Francisco. Reach him at anthony@venturebeat.com. (All story pitches should also be sent to tips@venturebeat.com) You can also follow Anthony on Twitter.

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