  We discuss the positivity of parton distribution functions using the common
$\overline{MS}$ factorization scheme. We find that in the perturbative regime
$\overline{MS}$ PDFs inherit the strict positivity of physical PDFs. We
explicitly discuss the scheme transformation by using suitable physical
observables and find that $\overline{MS}$ PDFs are positive above the scale
$Q^2$ > 5 GeV^2. Finally, we comment on the direct counterpart of
longitudinally polarized PDFs, finding agreement with the unpolarized
counterpart.
