It is postulated that the higher E3S uptake observed in the MCF-7 tumours is due to one or a combination of the following factors: higher affinity of MCF-7 cells for E3S, higher levels of OATP1A2 expression in MCF-7 xenografts and an increased sulphatase activity in MCF-7 cells. The gene discussed is SLCO1A2; the disease is neoplasm.