  In the recent article by Feng et al. [Science 333, 729-733 (2011)] a system
is proposed with a periodic spatial modulation of refractive index and
absorption. It is claimed that this system is nonreciprocal and can be used as
an optical isolator. The authors use an unconventional definition of
reciprocity and thus make wrong conclusions on the applicability of the
described phenomena for optical isolators. (Submitted to Science on October 7,
2011)
