key command "M" doesn't work in KEY EDITOR

Hello! I before worked in Cubase 4 and all functions worked very well but later in next versions of Cubase
I noticed some errors(
Why you cleaned the convenient Key commands? for example - key “M” doesn’t work in KEY EDITOR like mute track.
it became very inconvenient!
Please return and correct such errors with updating.

at fact this global ‘M’ is missing from version ‘6’ … I think