  We revisit our argument that shows that parton distribution Functions (PDFs)
in the MSbar{ scheme are non-negative in the perturbative region, with the main
goals of elucidating its domain of validity and clarifying its theoretical
underpinnings. We specifically discuss recent results proving that PDFs can
turn negative at sufficiently low scale, we clarify quantitatively various
aspects of our derivation of positivity in the perturbative region, and we
provide an estimate for the scale above which PDF positivity holds.
