Auto-tuning on iOS is a big moneyspinner.
iPhone music app developer Smule says its I Am T-Pain app has now sold more than two million copies on the App Store.
What's more, it's the second Smule app to pass the milestone, following earlier hit Ocarina.
Co-founder and CTO Ge Wang revealed the news at Billboard's Mobile Entertainment Live Music App summit in San Francisco.
I Am T-Pain is by far the most successful artist music app to have been released on Apple's App Store. It allows fans of T-Pain to sing along to his songs, and get their vocals auto-tuned.
Updates since its launch have added video, and the ability to sing along to other tracks - or simply freestyle a message to post on friends' Facebook pages.
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