Aim of the study: 70% of breast cancers (BC) are estrogen receptor alphapositive (ER+), and typically treated by targeting ER signaling. Previous research showed that membrane ER, as well as activation function 1 (AF-1) and activation function 2 (AF-2) domains are involved in the mammary gland and BC development. We aimed to characterize the contribution of each ER domain in ER regulated transcriptional profile in mammary epithelial cells (MEC). First, we researched whether ER-dependent gene expression is domain-specific. We tested the hypothesis that pathways relevant for BC are ER domain-dependent, and aimed to characterize whether those pathways are regulated by the cooperation of all domains or by an individual domain. We focused on the dependences of the expression of ER target genes, such as Areg, Greb1, and Pgr on different ER domains. Finally, we aimed to characterize the levels of interactions between PR and ER in cancer cell lines MCF7 and T47D. Material and methods: In this thesis, RNAseq data of GFP+ MECs from C57Bl6 mice lacking complete ER, membrane ER, AF-1 or AF-2 ER domains were analyzed in R software. MCF7 and T47D cell lines were treated with 100 nM progesterone (P4) to establish the conditions suitable for ER-PR interactions. The expression of Pgr was measured by qPCR and ER-PR interactions were analyzed by proximity ligation assay. Results and conclusions: Each ER domain strongly contributes to ER-regulated gene expression in MECs. To regulate pathways involved in carcinogenesis, cell proliferation, and immune response, ER domains act synergically, but also, in some pathways, an individual domain plays a regulatory role of the whole receptor. ER-PR direct interactions occur in nuclei, but also in non-nuclear compartments in MCF7 and T47D cells with an increased number of interactions upon P4 treatment.