MC through a variable trouble

hi, i´m having trouble trying to target a MC through a variable.
e.g.:I have 3 buttons: the MC for the button 1 is called btn1,for button 2 is btn2,and so on.Now I save the name of the MC in a variable called btn, but when I target the MC it doesn´t work…my button actions are like this:
btn starts as “”

Code:
if (btn != “”) {
btn.gotoAndPlay(32);
}
pictures.gotoAndPlay(2);

  btn = "pictures"; 

}

I´ve placed a trace(btn) and it looks fine, but when i used this code in other button and the MC pictures was loaded before, it doesn´t target the MC pictures…any ideas??