At a vascular permeability in the tumor of 4 × 10-7 cm/s (dotted lines), the free VEGF level in the available tumor interstitial fluid is approximately proportional to the secretion rate of VEGF in the tumor while the VEGF concentration in the blood plasma is rather insensitive to the change in the tumor VEGF secretion. The gene discussed is VEGFA; the disease is neoplasm.