  The sum of independent, but not necessary identically distributed,
exponential random variables follows hypoexponential distribution. We focus on
a particular case when all, but one rate parameters of the exponential
variables are identical. this is known as exponentially modified Erlang
distribution in molecular biology. We prove a characterization of the
exponential distribution, which complements previous characterizations via
hypoexponential distribution with all rates different from each other.
