EGFR/MAPK signalling has been implicated in mediating tamoxifen-resistant breast cancer cell growth in the clinic and in preclinical models. However, ERa expression and functionality has also been shown to be maintained in this condition. ERa transcriptional activity can be driven, in a tigand-independent manner, via growth factor signalling-mediated phosphorylation of ERa. The aim of mis thesis was to investigate whether growth factor signalling pathways regulate phosphorylation and functionality of ERa in tamoxifen-sensitive (WT) and -resistant (TAM-R) MCF-7 breast cancer cell lines and if so whether mis cross-talk mechanism plays a role in the generation and maintenance of the tamoxifen-resistant phenotype. Western blotting and immunocytochemistry assays revealed increased levels of serine 118 (SI 18), but not serine 167, phosphorylated ERa in TAM-R compared to WT cells. Basal SI 18 ERa phosphorylation was regulated by both EGFR and IGF-IR signalling pathways, via MAPK, in Tam-R cells and by IGF-1 R/phosphatidylinositol 3-kinase signalling in WT cells. ERa transcriptional activity, assayed by oestrogen response element (ERE) activity and pS2 and amphiregulin (AR) mRNA levels, was similarly IGF-1R/EGFR/MAPK-regulated in TAM-R cells, whereas, ERE activity was only IGF-1R-dependent in WT cells. AP-1 and serum response element activity was EGFR/IGF-1R-independent in born cell lines. Recruitment of the co-activators p68 RNA helicase and SRC1 was EGFR/MAPK- and SI 18 phosphorylation-dependent in TAM-R cells indicative of a role for SI 18 phosphorylation in mediating ERa transcriptional activity. The ability of ERa to regulate AR mRNA expression also suggested the existence of a self propagating autocrine growth regulatory loop in TAM-R cells. This was confirmed by the presence of ERa on the AR gene promoter, elevated basal AR mRNA expression, inhibition of EGFR, MAPK and ERa SI 18 phosphorylation by AR neutralising antibodies and growth promotion by AR and inhibition by the selective EGFR tyrosine kinase inhibitor gefitinib and the pure antioestrogen fulvestrant.