Marc Andreessen, the Web pioneer who co-founded Netscape Communications more than two decades ago, has become one of the most prominent investors in Silicon Valley.

His venture-capital firm, Andreessen Horowitz, was among the early investors in Twitter Inc. and Facebook Inc., bet on Skype ahead of its acquisition by Microsoft Corp. and more recently has become a leading evangelist for bitcoin, investing in the digital-currency arena and tweeting and blogging about its potential.

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