  In the early 1980s, Mills, Robbins and Rumsey conjectured, and in 1996
Zeilberger proved a simple product formula for the number of $n \times n$
alternating sign matrices with a 1 at the top of the $i$-th column. We give an
alternative proof of this formula using our operator formula for the number of
monotone triangles with prescribed bottom row. In addition, we provide the
enumeration of certain 0-1-(-1) matrices generalizing alternating sign
matrices.
