Gamma-ray emitting narrow-line Seyfert 1 galaxies. New discoveries and
  open questions
  The discovery of gamma-ray emission from 5 radio-loud narrow-line Seyfert 1
galaxies revealed the presence of a possible emerging third class of AGNs with
relativistic jets, in addition to blazars and radio galaxies. The existence of
relativistic jets also in this subclass of Seyfert galaxies opened an
unexplored research space for our knowledge of the radio-loud AGNs. Here, we
discuss the radio-to-gamma-rays properties of the gamma-ray emitting
narrow-line Seyfert 1 galaxies, also in comparison with the blazar scenario.
