Newbie: Unable to write a custom function

[deech]

Hi all,
I am new to Emacs. I have written a new search function into my .emacs
file but I am unable to invoke it with M-x when I restart Emacs. There
are no error messages when Emacs starts up. Here is the code:

;;Search backwards from a point for a string. If found delete
;;all characters from that string to the point.
;;Eg Given the argument 't', 'alligator' becomes 'alliga'
(defun isearch-backward-tophrase ()
  (set-mark-command ())
  (isearch-backward )
  (kill-region ()())
)

Any idea what I am doing wrong?

Thanks...
Deech