That's gotta hurt. After Affymetrix (Nasdaq: AFFX) lowered its 2008 revenue guidance by about 3% yesterday evening, to a range of $490 million to $510 million, its stock opened more than 35% lower today. I guess in this market, any perceived weakness is an excuse to cut and run.

The Motley Fool Rule Breakers pick also announced estimated first-quarter revenue of $80 million, excluding the $90 million payment from Illumina (Nasdaq: ILMN) to settle the companies' lawsuit. That means revenues were essentially flat for the first quarter year over year. And that's not something a company can get away with if, like Affymetrix yesterday, its trailing price-to-earnings ratio is approaching 100.

The biochip maker blamed the fall in revenues on decreased spending on research and development among pharmaceutical companies. The R&D drop shouldn't be a big surprise, given all of the layoffs we've been hearing about in recent weeks from the likes of Wyeth (NYSE: WYE) and Schering-Plough (NYSE: SGP). And it certainly doesn't bode well for other companies, such as Invitrogen (Nasdaq: IVGN) and QIAGEN (Nasdaq: QGEN), which sell laboratory supplies to companies.

Affymetrix's biggest weakness is that its revenue stream isn't terribly diversified -- even its most recent acquisition of USB didn't make it less dependent on companies and universities purchasing laboratory supplies, although the purchase did broaden its offerings. It's supplying its machines to Laboratory Corp. of America (NYSE: LH) to provide clinical diagnostics and is developing companion genetic tests to be used in combination with drugs, but Affymetrix hasn't come close to reaching its full potential in clinical diagnostics. In a few years, Affymetrix should be able to ride the R&D spending waves much more easily.

Investors will get to see full financial results and hear the company's plan for the future at next week's conference call. If it can hit its new revenue guidance for the year, it'll still have year-over-year revenue growth of 8%-13% excluding this year's one-time gain. If it can cut costs to make up the lower top-line growth, Affymetrix should be OK this year.

Affymetrix is a Rule Breakers pick. Our growth-stock newsletter service is always on the hunt for hot drug stocks and other cutting-edge picks. See all of our latest discoveries with a free 30-day trial subscription.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Laboratory Corp. of America is a Motley Fool Stock Advisor pick. The Fool has a disclosure policy.