You release 10⁹ protein molecules at position x = 0 in the middle of a narrow capillary test tube. The molecules’ diffusion constant is D = 10−6 cm2 s −1 . An electric field pulls the molecules to the right (increasing x) with a drift velocity of v = 1 µm s−1 . Nevertheless, after 80 s you see that a few protein molecules are actually to the left of where you released them. How could this happen? What is the ending number density right at x = 0? This is a one-dimensional problem.