  The magnetorotational instability (MRI) plays a crucial role for cosmic
structure formation by enabling turbulence in Keplerian disks which would be
otherwise hydrodynamically stable. With particular focus on MRI experiments
with liquid metals, which have small magnetic Prandtl numbers, it has been
shown that the helical version of this instability (HMRI) has a scaling
behaviour that is quite different from that of the standard MRI (SMRI). We
discuss the relation of HMRI to SMRI by exploring various parameter
dependencies. We identify the mechanism of transfer of instability between
modes through a spectral exceptional point that explains both the transition
from a stationary instability (SMRI) to an unstable travelling wave (HMRI) and
the excitation of HMRI in the inductionless limit. For certain parameter
regions we find new islands of the HMRI.
