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

Jan. 11

Weekly Loss

My Watchlist

Sequenom (NASDAQ:SQNM)

$4.23

19%

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Molycorp (NYSE:MCP)

$8.48

18%

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Apollo Group (NASDAQ:APOL)

$19.21

13%

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Universal Display (NASDAQ:OLED)

$25.20

11%

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Fusion-io (NYSE:FIO)

$19.93

10%

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Source: Barron's.

Sequenom slipped after posting preliminary financial results this past weekend. A presentation at a J.P. Morgan health-care investing conference on Wednesday and a rival's acquisition didn't help, as shares of the provider of diagnostic testing and genetic analysis solutions slipped in each of the first four days of the trading week.

Molycorp stumbled after revealing production delays at its Mountain Pass mine. As a result of the delay, the rare-earth elements provider had to slash its 2013 outlook. Investors went ahead and slashed their positions.

Apollo Group was the poster child of online education when for-profit post-secondary educators were all the rave, but the University of Phoenix parent posted problematic quarterly results. New student enrollments fell by 15%, leading the company to follow Molycorp in hosing down its outlook for the year ahead.

Universal Display was garnering analyst praise late last month. The pioneer in organic light-emitting diode displays had Cowen & Co. and JPM Securities issuing bullish notes in late December on Universal's prospects of being a star during this past week's Consumer Electronics Show. It wasn't. The stock slipped accordingly.

Finally, we have Fusion-io losing ground after an analyst cooled on the company. William Blair analyst Jason Ader lowered his rating from "outperform" to "market perform" on fears that enterprise bookings growth is slowing as companies scale back their IT spending habits.

Longtime Fool contributor Rick Aristotle Munarriz has no position in any stocks mentioned. The Motley Fool recommends Universal Display and owns shares of Fusion-io and Universal Display . Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.