HTC One M9 source code published online4
The HTC One M9 kernel source code is based on Android 5.0.1 Lollipop, which is the exact same version of Android that the flagship currently runs (makes sense, doesn't it). There are multiple localized versions to choose from, ranging from the Orange Spain version, to the unlocked European version, and the Vodafone Germany version. All versions clock in at exactly 517MB. At the moment, there's no US-specific version of the kernel source code to download, but chances are that more versions will be added in the future.
As mentioned above, the reason why publicly-available kernel source codes are important is because developers can use them to create custom Android MODs. Here are some of the best Android MODs developed for last year's HTC One M8. For a detailed analysis of the latest and greatest HTC flagship smartphone, make sure to check out our in-depth HTC One M9 review. We also have you covered if you'd rather learn more about the new features in the HTC Sense 7 UI.
source: HTC Developers