San Ramon, California (US): Apple Inc.’s Chief Executive Officer Tim Cook will take the witness stand Friday to defend the company’s iPhone App Store against charges that it has grown into an illegal monopoly—one far more profitable than his predecessor Steve Jobs envisioned when it opened up 13 years ago.

The technology company is counting on Cook’s appearance to put the finishing touches on Apple’s defense against an antitrust case brought by Epic Games Inc., the maker.

Epic Games is trying to prove that the store has morphed into a price-gouging vehicle that not only reaps a 15% to 30% commission from in-app transactions but blocks third-party apps from offering other payment alternatives. That extends to just showing a link that would open a web page offering commission-free ways to pay for subscriptions, in-game items, and the like.

Apple fiercely defends the commissions as a fair way for app makers to help pay for innovations and security controls.