# File format of substitution matrix in clustalw

I need to set the substitution matrix used by command line CLUSTALW when comparing DNA sequences to:

 0 -1 -1 -1
-1  0 -1 -1
-1 -1  0 -1
-1 -1 -1  0


from my understanding I need to pass the location of a file with the matrix to the -dnamatrix parameter, however, I cannot find out what format the file is supposed to be in. Any idea?

   A   G   C   T   U   *