Marissa Mayer signed on to be chief at Yahoo! Inc. (NASDAQ:YHOO) exactly one year ago today. Fortuitously the anniversary coincides with the company announcement of its earnings for the second quarter of 2013. Mayer has managed to surprise Wall Street so far in her reign at the head of the company, but can she continue to do so this afternoon?

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First let’s look at the expectations she’s supposed to beat. Wall Street analysts are looking for earnings of 31 cents per share from Yahoo! Inc. (NASDAQ:YHOO) on revenue of $1.1 billion. Every quarter since she got the job Mayer has managed a beat on earnings. Revenue has remained flat, however, and that’s probably more important in the medium term.

Yahoo acquisition trouble

Mayer has been congratulated for her unflinching and confident acquisitions. Since taking over Yahoo! Inc. (NASDAQ:YHOO), Mayer has added Tumblr, Qwiki and Xobni among others. The key strategy behind these acquisitions is diversification. Unfortunately, Yahoo has simply managed to diversify its assets while leaving its revenue reliant on its older core business.

Investors know that these acquisitions are going to cost money in the short term, but they’re expecting some kind of plan regarding their monetization to come through in the near future. Mayer is being given a huge amount of rope, all of her investors hope she knows what to do with it.

Old business isn’t bad business

Yahoo has been living off of the revenue brought in by its core web portal for years. Despite its decline, the company has been able to rely on the revenue from services like Yahoo News to keep it afloat, even as those revenue streams continued to atrophy. Mayer has managed to freeze that recession, but she has not yet managed to regrow the core business of Yahoo in a meaningful way.

It’s clear that Mayer isn’t going to let Yahoo’s core business die. She needs the revenue it provides in order to fund her transformation of the company. Investors know this, and they’ll be looking at the returns from Yahoo! Inc. (NASDAQ:YHOO)’s core business in this afternoon’s earnings report.

Yahoo stock movement

Since the start of 2013 Yahoo! Inc. (NASDAQ:YHOO) has increased in value by more than 36 percent. Since Mayer took over at the helm of the company, its stock has risen by more than 70 percent. The firm’s P/E ratio now stands at just under eight. Yahoo! Inc. (NASDAQ:YHOO) is by no means overvalued.

The reason for the company’s low valuation is the fear that core business could recede faster than anticipated. If today’s figures show that process starting, stock is likely to fall in proportion after close. If core business remains strong on the other hand, it removes the company further from the angst of recent years. That should have a positive impact on the stock price.

Yahoo! Inc. (NASDAQ:YHOO) will announce its earnings after the market closes. Look out for core performance. At this stage it’s about all that matters at the company.