The Yamaha MU-series is a line of sound modules built by Yamaha. All sound modules except MU5 support Yamaha XG. The sound modules were commonly used when computers had slower processors. The computer could send MIDI commands to the sound module, acting as an external sound generation device. Later MU sound modules feature A/D inputs that allow direct input from microphones and guitars.
The MU-series product line superseded the company's previous TG-series modules, the TG100 and TG300. Although the majority of Yamaha's MU-series modules were meant for the home user, the company also made rack-mount versions of the MU90 and MU100 called the MU90R and MU100R, respectively, for professional use.
Read more about Yamaha MU-series: List of MU-series Sound Modules