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Human endometrium has been extensively investigated in the search for markers capable of predicting its receptive status. The completion of the Human Genome Project has triggered a rapid development of new fields in molecular biology, the "transcriptomics" being a major turning point in the knowledge acquisition of endometrial receptivity. Based on this, a customized Endometrial Receptivity Array (ERA) has been developed, which is capable of identifying the genomic signature of receptivity. This diagnostic tool showed that the window of implantation (WOI) is displaced in one out of four patients with implantation failure, allowing the identification of their personalized WOI. This strategy allows performing a personalized embryo transfer ( pET) on the day in which the endometrium is receptive. The combination of a systems biology approach and next-generation sequencing will overcome the limitations of microarrays, and will, in the future, allow elucidation of the mechanisms involved in embryo implantation.T he endometrium, which lines the inside of the uterine cavity, undergoes cyclic changes that are regulated by ovarian steroids. It can be subdivided into the basal layer that is responsible for its regenerative capacity, and the functional layer that undergoes proliferation, secretion, and tissue degeneration every month from menarche to menopause. Its aim is to prepare the optimal moment for embryonic implantation known as the window of implantation (WOI).The endometrial cycle comprises the menstrual, proliferative, and secretory phases. The proliferative phase, which corresponds to the ovarian follicular phase and increased production of estrogens, lasts until ovulation occurs. During this stage, the increasing estrogen levels cause the proliferation of the stromal cells as well as the elongation of the spiral arteries. After ovulation, with the appearance of progesterone secreted by the granulosa-luteal cells, the secretory phase begins. If implantation does not occur, the secretory phase ends, and the corpus luteum degenerates. Menstruation occurs owing to the drop of estrogen and progesterone, which resets the endometrium until pregnancy

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Many women with endometriosis experience compromised fertility. This disease clearly exerts quantitative damage on the ovaries, and perhaps, also qualitative damage. However, it remains controversial whether endometrial receptivity is compromised. Here we review the evidence from basic transcriptomic signature data to clinical data from an oocyte donation model and find support for the concept that endometrial receptivity is not impaired in women with endometriosis when healthy embryos reach the endometrial cavity.

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