  The origin of radio emission in the majority of Active Galactic Nuclei (AGN)
is still poorly understood. Various competing mechanisms are likely involved in
the production of radio emission and precise diagnostic tools are needed to
disentangle them, of which variability is among the most powerful. For the
first time, we show evidence for significant radio variability at 5 GHz at
milli-arcsecond scales on days to weeks time scales in the highly accreting and
extremely radio-quiet (RQ) Narrow Line Seyfert 1 (NLSy1) Mrk110. The
simultaneous Swift/XRT light curve indicates stronger soft than hard X-ray
variability. The short-term radio variability suggests that the GHz emitting
region has a size smaller than ~180 Schwarzschild radii. The high brightness
temperature and the radio and X-ray variability rule out a star-formation and a
disc wind origin. Synchrotron emission from a low-power jet and/or an
outflowing corona is then favoured.
