Shimano Ultegra RD-R8150 Di2 12-Speed Rear Derailleur E-tube Fitting:

Ultegra Shadow design with Di2 technology, shifting is easy and precise - just a light touch, as soft as a mouse click, is sufficient for the front or rear derailleur to instantaneously select the correct gear The rear derailleur is now the brains of the system with wireless connectivity to the shifters. It acts as the point the system is charged, provides ANT+ connection to 3rd party devises and replaces the handlebar junction Button on the derailleur for Di2 operations, and LED light denotes synchronized shift mode, setting mode and charge requirements Sleek appearance with Low profile Shadow design construction reduces risk of damage in a crash Programmed automatic motion at the touch of a switch makes this derailleur quick, smooth and effortless to shift The removal of traditional mechanical cables in favour of electrical wires not only eliminates poor shifting caused by contaminates, corrosion and stretching, but provides effortless gear shifts Derailleur contains a saver (fuse) f