We have recently put a manuscript on bioRxiv:
A Facilitated Diffusion Mechanism Establishes the Drosophila Dorsal Gradient http://biorxiv.org/content/early/2016/06/03/057091
In our manuscript, we have found that the Dorsal gradient is, at least in part, established by a facilitated diffusion mechanism. This is a way that the Dorsal protein can fight against its concentration gradient and accumulate on the ventral side of the embryo. Without this, the Dorsal gradient likely would not get "tall" enough to specify all of the necessary genes, such as snail.
If you haven't checked out bioRxiv, it is the preprint server for
biology hosted by Cold Spring Harbor Laboratory. This is where
researchers will post their findings before they get officially
published in a peer-reviewed journal.