Am I missing Mute Region?

Just come back to Pro Tools after some years in the wilderness and just started editing in First, great system, just can't find how to mute a region. Lists of Pro Tools short cuts show it as (on a a Mac) Command+M which is what I remember from way back but it does not seem to work in First. Is this a limitation of First or am I doing something wrong? :-)

Thanks

G

Sys is 10.11.1 Mac Mini 2012 i7 4GB (all possible OS and App updates run)

Am I missing Mute Region?