**5. Conclusion**

This chapter summarised the current knowledge of E2 and P4 action in human endometrium and mammary gland. To understand both sides of steroid hormone action - in normal physiology and especially in pathology it is important to understand the molecular intracellular events of E2 and P4 in tissue-type manner. The same hormones could have different or even opposite effects in different tissues. Thus, attention should be applied to steroid hormones' or their analogues' possible side effects before using them in clinical treatments. Questions still remain about the aberrations of the endometrium leading to implantation failure, endometriosis and other dys-regulations. Even though studies of breast cancer development continuously unravel new information about the mechanisms leading from normal to malignant tissue proliferation, breast cancer is still the most fatal cancer type among women. The number of couples seeking for aid to achieve fertility is constantly increasing and thus a better understanding of factors needed for successful treatment and possible side effects is crucial.

We would like to punctuate the importance of the next generation sequencing technologies which we believe are the key in understanding hormone dependent action in whole organism. With today´s knowledge of nearly entire human genome sequence and the development of new technologies based on HTP sequencing it has become possible to define all targets for the TF in vivo and establish entire transcriptome in a single experiment. Data analysis is still complicated in a way and needs excellent computational skills but the data collected today will become the knowledge of tomorrow.
