system
A chat on subreddit /r/pcmasterrace
human
No, no, no. ATX, mATX, ITX are all backwards compatible. You can fit an ITX motherboard in a giant Corsair 7000D and the PCI-E slot will line up exactly with the case slots.

If your case supports ATX motherboards it will fit *every* mATX and ITX motherboard and if it doesn't then you have installed it incorrectly. Which OP did.
gpt
Perfect.