The upper limit (around -3.9) corresponds to the diffusion limit through an unstirred water layer (UWL), as explained above. Here, it can be noted that at least 40% of molecules studied using CACO-2 or MDCK assays were close to this diffusion limit. Therefore, a significant amount of measured permeability is, in fact, diffusion of the unstirred water layer, and their intrinsic permeability is thus anywhere between -4 and 4. However, for example Stenberg et al, published CACO-2 data where they tried to reduce the effect of UWL on permeability by stirring .