Cancer is the number one cause of mortality worldwide. Breast cancer is the most common type of cancer diagnosed in both sexes and the major cause of cancer-related mortality in women. The vast majority of breast tumors (~70%) express estrogen receptor α (ERα) and depend on estrogen for their growth. ERα is a hormone receptor and when it binds to its ligand (estrogen), it can promote tumor progression as a transcription factor. ERα-mediated growth is also possible in the absence of estrogens, through direct or indirect activation of ERα by kinases such as the mitogen-activated protein kinases (MAPKs) ERK1/2. Owing to the strong dependency on the estrogen-ERα axis, targeting ERα with antagonists, for example tamoxifen or fulvestrant, or inhibiting the production of estrogens with aromatase inhibitors have been therapeutic approaches for ERα+ breast tumors for decades. Tamoxifen has now been successfully used in the clinical setting for over 40 years to treat ERα+ tumors. However, around one third of the patients treated with tamoxifen ultimately develop resistance and face recurrence of the disease. In order to overcome this challenge, it is very important for new biomarkers and potent therapeutic targets to emerge for the early detection of tamoxifen-resistance and ultimately for most efficient treatment. For this reason, a whole-genome CRISPR/Cas9 knockout screen was conducted in our laboratory to identify possible new biomarkers of tamoxifen resistance as well as novel therapeutic targets for breast cancer. Seven genes, namely FIBP, STK11, PTPN12, SPRED2, UBE2R2, CUX1 and LZTR1, whose depletion yielded a similar profile of tamoxifen- and fulvestrant-resistant phenotypes, were chosen for further validation in this project. Later on, we decided to focus on SPRED2 for a more in-depth investigation of the underlying molecular mechanisms that might confer tamoxifen resistance after its depletion. We found that SPRED2 deficiency leads to hyperactivation of ERK1/2, which in turn leads to an increase in ERα activity and eventually resistance to tamoxifen. In this study, we propose a therapeutic regimen of ERK1/2 inhibitor, ulixertinib, in combination with tamoxifen for the treatment of those ERα+ breast cancers characterized by low expression levels of SPRED2. Identification of SPRED2 as novel biomarker of tamoxifen resistance brings us closer to personalized medicine, where treatments are chosen based on the genetic signature of each individual patient.