TechPowerUp posted a review on the AMD Ryzen Threadripper Motherboards to be Showcased on July 25th
A quote from the article:
AMD is organizing the "Meet the Experts" webinar, which will focus on AMD's pcoming Ryzen Threadripper motherboard designs and offerings from AMD partners. As we inch closer to AMD's HEDT X399 platform launch, we've gotten confirmation from AMD on Threadripper's specs and pricing. However, the actual motherboards where you're expected to sit your awe-inducing 12 and 15-core processors have largely been absent from the show.AMD Ryzen Threadripper Motherboards to be Showcased on July 25th @ TechPowerUp
And since AMD knows that processors without a motherboard don't really equate to anything much, the company has invited ASUS, Gigabyte, MSI and ASRock to detail at least some of their X399 motherboards. So far, the motherboards we have some info are the GIGABYTE X399 AORUS Gaming 7 (which has 5x PCIe x16 slots, no PCIe x1 slots, and 3x M.2 slots in an ATX form-factor); the ASUS X399 ROG ZENITH EXTREME (EATX, 4x PCIe x16 slots, 1x PCIe x1 slots, and 2x M.2 slots); the ASROCK X399 Professional Gaming (ATX, 4x PCIe x16 slots, 1x PCIe x1 slots, and 2x M.2 slots); and finally, the ASROCK X399 TAICHI, which counts with the usual ATX form-factor, and offers 4x PCIe x16 slots, 1x PCIe x1 slots, and 2x M.2 slots. All of these seem to be marketed toward gamer enthusiasts, though we'll see some increasingly workstation-geared motherboards closer to or after the launch.