“…However, it seems likely that it is the level of oestrogen in the tissue, rather than that in the plasma which ultimately influences ER levels. In tumour tissue, the endogenous oestrogens derive not only from the plasma, but also from biosynthesis within the tumour (Miller & Forrest, 1974;Miller et al, 1981;Mason et al, 1981) and hydrolysis of oestrone sulphate (Santner et al, 1984); oestradiol rather than oestrone appears to be the major free oestrogen (Poortman et al, 1983). In general, in tumour tissue, oestrogen concentrations are likely to be higher when ER are present than when they are absent (Fishman et al, 1977;Edery et al, 1981;Thorson et al, 1982;Drafta et al, 1983), and in one study (Edery et al, 1981), tumour oestradiol content rose with increasing ER level.…”