  The detection by Fermi-LAT of gamma-ray emission from radio-loud Narrow-Line
Seyfert 1s (NLS1s) indicates that relativistic jets do not form only in blazars
and radio galaxies, but also in other AGN populations. Despite a spectral
energy distribution similar to blazars, their physical characteristics are
quite different: lower black hole masses, generally higher accretion rates, and
possibly hosted in spirals. Furthermore, their radio properties make the
interpretation of these objects even more puzzling. The radio emission is very
compact, not exceeding the parsec scales, as also found in the population of
young radio sources. We present high resolution VLBA observations of three
radio-loud NLS1s detected by Fermi-LAT: SBS 0846+513, PKS 1502+036, and PKS
2004-447. The information on the pc-scale morphology will be complemented with
studies of flux density and spectral variability from multi-epoch and
multifrequency observations, in order to unveil the nature of their radio
emission.
