  Let $M$ be a perfect matching in a graph. A subset $S$ of $M$ is said to be a
forcing set of $M$, if $M$ is the only perfect matching in the graph that
contains $S$. The minimum size of a forcing set of $M$ is called the forcing
number of $M$. Pachter and Kim [Discrete Math. 190 (1998) 287--294] conjectured
that the forcing number of every perfect matching in the $n$-dimensional
hypercube is at least $2^{n-2}$, for all $n \ge 2$. Riddle [Discrete Math. 245
(2002) 283-292] proved this for even $n$. We show that the conjecture holds for
all $n \ge 2$. The proof is based on simple linear algebra.
