  Recently, Brodsky, Hwang and Schmidt have proposed a new mechanism that gives
a transverse spin symmetry at leading twist in semi-inclusive deep-inelastic
scattering. I show that the new mechanism is compatible with factorization and
is due to an transverse-spin asymmetry in the k_T distribution of quarks in a
hadron (the "Sivers asymmetry"). An earlier proof that the Sivers asymmetry
vanishes because of time-reversal invariance is invalidated by the path-ordered
exponential of the gluon field in the operator definition of parton densities.
Instead, the time-reversal argument shows that the Sivers asymmetry is reversed
in sign in hadron-induced hard processes (e.g., Drell-Yan), thereby violating
naive universality of parton densities. Previous phenomenology with
time-reversal-odd parton densities is therefore validated.
