  We argue that the 21-cm global signal can be a powerful probe of isocurvature
perturbations, particularly for the ones with blue-tilted spectra. Although the
21-cm global signal is much affected by astrophysical processes, which give
some uncertainties when cosmological models are investigated, recent results
from HERA have constrained several astrophysical parameters, whose information
can reduce the ambiguities originating from astrophysics. We show that the size
and spectral tilt of isocurvature perturbations can be well inferred from the
21-cm global signal once the information on astrophysics from the HERA results
is taken into account.
