  Before the launch of the Fermi satellite only two classes of Active Galactic
Nuclei (AGN) were known to generate relativistic jets and thus to emit up to
the gamma-ray energy range: blazars and radio galaxies, both hosted in giant
elliptical galaxies. The first four years of observations by the Large Area
Telescope (LAT) on board Fermi confirmed that these two populations represent
the most numerous identified sources in the extragalactic gamma-ray sky, but
the discovery of variable gamma-ray emission from 5 radio-loud Narrow-Line
Seyfert 1 (NLSy1) galaxies revealed the presence of a possible emerging third
class of AGN with relativistic jets. Considering that NLSy1 are thought to be
hosted in spiral galaxies, this finding poses intriguing questions about the
nature of these objects, the knowledge of the development of relativistic jets,
and the evolution of radio-loud AGN. In this context, the study of the
radio-loud NLSy1 from radio to gamma-rays has received increasing attention.
Here we discuss the radio-to-gamma-rays properties of the gamma-ray emitting
NLSy1, also in comparison with the blazar scenario.
