Referencing with variables?

Is there a way to set a variable for referencing an mc. Like when you reach a certain frame you set a variable:

_global.currentMC = "lsnKeyTerms";

And then on a button:

on (release) {
_root.lessons_mc._global.currentMC.goBackwards(15);
}

or can you say something like

on (release) {
currentlyplaingmc.goBackwards(15);
}

?