Amazon’s $400 Billion Rally Hinges on Cloud Results Inc.’s run as one of the best stocks this year will likely come down to the performance of a single business line: cloud computing.

That’s the lesson, at least, from the results delivered by Alphabet Inc. and Microsoft Corp. this week. The Windows software maker rode its Azure cloud-services business to a 3.1% gain on Wednesday, while Google’s parent suffered its biggest stock drop in more than three years after cloud sales marred otherwise strong results. Amazon sank 5.6% in a broad tech rout ahead of its earnings due on Thursday afternoon.

The divergence shows how central cloud computing has become for the tech behemoths following a rally this year that stretched valuations, contributing to an environment where any disappointments are being punished severely.

“I am a bit more nervous about Amazon now,” said Dan Eye, chief investment officer at Fort Pitt Capital Group. “Given where tech valuations are, you have to be selective. I’ve always been a bit cautious on Amazon since it runs such thin margins, and it seems fair to think that if it is also seeing some weakness with its cloud, that will really show up in its operating margins.”

Amazon is Still Pricey