There's never a shortage of losers in the stock market. Let's take a closer look at five of this past week's biggest sinkers.

Company

May 17

Weekly Loss

Oncothyreon (NASDAQ:ONTY.DL)

$1.99

21%

Cliffs Natural Resources (NYSE:CLF)

$20.69

12%

Molycorp (NASDAQOTH:MCPIQ)

$6.52

11%

Taseko Mines (NYSEMKT:TGB)

$2.10

11%

Velti (NASDAQOTH:VELTF)

$1.82

10%

Source: Barron's.

Let's start with Oncothyreon, which took a hit after offering uninspiring clinical data for Stimuvax. The biotech and its investors had big hopes for the experimental lung-cancer treatment, but its latest study didn't meet Oncothyreon's goal of materially improving the overall survival rate.

Cliffs Natural Resources saw its shares soar 18% a week earlier after receiving a timely FBR Capital analyst upgrade, but this week the iron ore miner resumed its losing ways. Fears of diminishing demand -- particularly as China's once smoking-hot economy slows -- are weighing on the shares. The stock has shed nearly half of its value in 2013.

Molycorp tumbled on a week that slammed most commodity stocks. The volatile rare-earth minerals miner had moved sharply higher earlier this month after delivering a much smaller quarterly deficit than analysts were expecting, but Molycorp joined other mineral miners on the way down this week.

Taseko Mines slipped earlier this month after posting a quarterly loss and announcing that its CFO was leaving. This week's slide was related to the continuing slump in metal prices, and a move on Friday to initiate a shareholder-rights plan did little to lift the shares of the Canadian copper miner.

Velti fell after the Dublin-based mobile marketing services provider posted disappointing quarterly results. Revenue fell 21% in the latest quarter. Wall Street knew it was going to be a bad quarter. Velti had let go a fifth of its staff earlier this year, bracing for a slowdown. However, Velti's guidance wasn't very encouraging.

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