Considering the fact that there was no significant difference in Ki67 index between the two tumour models at baseline, a second hypothesis to explain the similar tumour uptake of both radiotracers could be that the uptake of RGD tracers is related not only to microvasculature density, as confirmed by its significant correlation with the expression of CD31, but also to the overall proliferative capacity of the tumour. The gene discussed is PECAM1; the disease is neoplasm.