Back button to middle mouse button

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http://unix.stackexchange.com – I would like to map "Back" button (xev shows it as "button 7") of my mouse (A4Tech X-7) to act as a middle mouse button. Is there a way to do this? I'm lost in google, finding only how to map buttons to keyboard shortcuts with xbindkeys and etc. OS: Centos 6.3 (x86_64) DE: KDE 4.3.4 P.s. I can switch (with xmodmap) "Back" button with middle button with no problems, but is there a way to have both them act as a middle mouse button? (HowTos)