  We find that the recently developed kinetic theories with spin for massive
and massless fermions are smoothly connected. By introducing a reference-frame
vector, we decompose the dipole-moment tensor into electric and magnetic dipole
moments. We show that the axial-vector component of the Wigner function
contains a contribution from the transverse magnetic dipole moment which
accounts for the transverse spin degree of freedom (DOF) and vanishes smoothly
in the massless limit. As a result, the kinetic equations, describing four DOF
for massive fermions, becomes smoothly the chiral kinetic equations describing
two DOF in the massless limit. We also confirm the small-mass behavior of the
Wigner function by explicit calculation using a Gaussian wave packet.
