  We present the discovery of IGR J18245-2452, the first millisecond pulsar
observed to swing between a rotation-powered, radio pulsar state, and an
accretion-powered X-ray pulsar state (Papitto et al. 2013, Nature, 501, 517).
This transitional source represents the most convincing proof of the
evolutionary link shared by accreting neutron stars in low mass X-ray binaries,
and radio millisecond pulsars. It demonstrates that swings between these two
states take place on the same time-scales of luminosity variations of X-ray
transients, and are therefore most easily interpreted in terms of changes in
the rate of mass in-flow.
  While accreting mass, the X-ray emission of IGR J18245-2452 varies
dramatically on time-scales ranging from a second to a few hours. We interpret
a state characterised by a lower flux and pulsed fraction, and by sudden
increases of the hardness of the X-ray emission, in terms of the onset of a
magnetospheric centrifugal inhibition of the accretion flow. Prospects of
finding new members of the newly established class of transitional pulsars are
also briefly discussed.
