What happened

Shares of Guardant Health (NASDAQ:GH) were slipping Friday after investors noticed that one of the company's independent directors sold off some shares. The medical diagnostics provider's stock was down 9.4% as of 1:10 p.m. EDT, washing away the gains produced by the exciting second-quarter earnings report it delivered earlier this month.

So what 

Ian Clark was CEO of Genentech from 2010 through 2016, and now he sits on boards of a handful of companies, including Guardant Health. On Monday, Clark sold some shares of Guardant Health at $94.05 immediately after exercising an option that allowed him to acquire them for $4.18 per share.

A very unhappy stock trader.

Image source: Getty Images.

Clark disposed of just 538 shares, netting himself just $48,350 before taxes. That's hardly enough to raise alarms, but he also sold 9,683 shares of Guardant Health on the day of the company's second-quarter earnings call, just a few weeks ago.

Now what

When it comes to interpreting insider transactions, always remember that there are endless reasons to sell a stock, but just one reason to buy. Clark probably just needed the money for back-to-school shopping or some other innocuous purpose.

The reason he chose to sell significant shares of Guardant rather than of the other companies he works for looks pretty straightforward as well. Clark's also a director at Forty-Seven and Corvus Pharmaceuticals, shares of which have lost 51% and 58%, respectively, over the past year. Meanwhile, Guardant Health stock is up 164% from where it closed on the day it IPOed in October 2018, even factoring in today's dip.

While it's tempting to sell stocks that are underperforming, that's a bad idea 99% of the time. And taking profits is a less insidious motivation than today's stock flop would indicate. Unless Clark's other stock options increase in value, investors can expect more Guardant Health stock sales from this insider in the near future.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.