  A sign pattern matrix is a matrix whose entries are from the set $\{+,-,0\}$.
If $A$ is an $m\times n$ sign pattern matrix, the qualitative class of $A$,
denoted $Q(A)$, is the set of all real $m\times n$ matrices $B=[b_{i,j}]$ with
$b_{i,j}$ positive (respectively, negative, zero) if $a_{i,j}$ is +
(respectively, $-$, 0). The minimum rank of a sign pattern matrix $A$, denoted
$\mr(A)$, is the minimum of the ranks of the real matrices in $Q(A)$.
Determination of the minimum rank of a sign pattern matrix is a longstanding
open problem.
  For the case that the sign pattern matrix has a 1-separation, we present a
formula to compute the minimum rank of a sign pattern matrix using the minimum
ranks of certain generalized sign pattern matrices associated with the
1-separation.
