E*thirteen has launched its new XC-oriented chain retention system, called the XCX. As the name implies, it's designed for use on both XC mountain bikes and cyclocross bikes, allowing a simplified single-chainring transmission without the risk of dropping the chain and without the heft of existing freeride or DH devices.
According to e*thirteen's press release, the XCX is the first such gizmo to be ridden to a podium place at an "international World Cup XC event", although the race to which they refer is actually the 2009 World Championships (at which Julien Absalon came second with an XCX on his otherwise SRAM XX-equipped bike, although his name's not in the press release so presumably he didn't have a contract in place or something...), which isn't a World Cup event. It's not true anyway - Geoff Kabush used the conceptually-similar MRP 1.x device when he won the Bromont World Cup round a few weeks before the World Championships.
But enough tedious pedantry. The XCS is designed to be used with a single front chainring and either a nine or 10-speed cassette. It's adjustable for use with rings between 32 and 42 teeth and mounts to the bottom bracket. The backplate is 1.5mm thick and you get 0.5mm spacers in the box, so it should be straightforward to set up on most outboard BB systems. BB30 users will be out of luck, though.
Claimed weight is 56g, which is usefully lighter than two chainrings, a mech and a shifter. We expect to see a lot more 1x9/1x10 transmissions in use on MTBs, and not just for racing - with an 11-34 cassette you can get enough range for most riding (especially if you favour trail centres) with only a middle ring up front. The introduction of lightweight chain guides should boost the popularity of the setup.