  The absolute value squared of the probability amplitude corresponding to the
overlap of an initial state with a continuum wave solution to the Schr\"odinger
equation of the problem, has the physical interpretation provided by the Born
rule. Here, it is shown that for an open quantum system, the above probability
may be written in an exact analytical fashion as an expansion in terms of the
non-Hermitian resonance (quasinormal) states and complex poles to the problem
which provides an underlying non-Hermitian character of the Born rule.
