HTC One M7 UL (PN07120) - AT&T

Overview
The PN07120 model appears to be targeted for usage with 

This device supports single-stream 802.11abgn/ac
 * It is presumably using Broadcom BCM4335


 *  HTC One variants U/UL/WL on xda-developers forums
 * XDA wiki
 * XDA forums

To access the "HTC Function Test" (hardware troubleshooting),
 * dial: *#*#3424#*#*

Teardown

 * HTC One Teardown on iFixit
 * Inside the HTC One on Chipworks
 * CPU: Qualcomm APQ8064 (Snapdragon 600)
 * Flash: Samsung KLMBG4GE2A 32GB (eMMC NAND)
 * RAM: Elpida BA164B1P-1D-F 2GB (DDR2)
 * Wi-Fi: Broadcom BCM4335 (BCM4335XKUBG)

Access Developer Options Menu
The Developer options menu is by default hidden on the M7.

To enable USB debugging and more, you'll have to reveal this menu
 * with the steps outlined HTC forums.

Access Bootloader Consistently
The standard way of getting to the bootloader menu (holding power
 * and volume up/down while powered off) doesn't work consistently.

Instead, while your device is on, in USB Debugging mode, and connected
 * to your PC, run `adb reboot-bootloader`

Root Information
Unlock bootloader (wipes storage) >> Flash custom recovery >>
 * Full system backup >> Install superuser app >> Go crazy


 * Excellent guide to getting root, using all legitimate code
 * CyanogenMod installation instructions. More concise
 * TWRP Recovery download page for HTC M7
 * CyanogenMod ROM
 * (non-free) Google Apps package
 * Before applying the superuser zip to get root, factory reset in recovery, and take a (compressed) Boot/Data/System backup
 * Koush's open source Superuser app is recommended
 * Find the zip on the Google Play description
 * For convenience, instead of copying to your phone's storage, sideload superuser.zip in TWRP


 * Good explanation of why you shouldn't accept OTA (over-the-air)
 * updates after flashing recovery: http://teamw.in/officialota


 * To turn off OTA notifications (if rooted): Freeze "Updater" app using
 * Titanium Backup Pro.

Getting back to stock
You may want to do this if you want an OTA update, are experience
 * software issues, or want to send phone back to factory.


 * Step 1: Download most recent full zip for your device/carrier/region
 * from the HTCdev downloads page
 * Should be over 1 GB (or 1,000,000,000 bytes) in size
 * Choose WWE for Developer/Unlocked/Google edition devices


 * Step 2: Restore RUU.zip over adb, as instructed in this XDA post