Around two thirds of breast tumors are characterized by the expression of estrogen receptor α and are therapeutically treated by blocking estrogen signaling. First line endocrine therapeutics are Tamoxifen which displaces estrogen form its receptor preventing receptor activation and aromatase inhibitors which prevent the formation of estrogen and thereby hormone-deprive the tumors. Therapy resistance remains a major clinical problem, especially for patients with late-stage diagnoses. Tumor heterogeneity may promote therapy resistance either through the selection of pre-existing rare clones or by providing a repertoire of cells that may develop resistance over time. In order to study endocrine therapy resistance on a clonal level, we have developed barcoded (allowing the tracking of single cells) endocrine therapy sensitive and resistant breast cancer cell lines. From these complex cell pools, multiple single cells characterized by a specific barcode were isolated and grown out. We then subjected the clones to phosphoproteomics measurement by Mass spectrometry. Based on their phosphorylation pattern, the kinase activity profiles of endocrine therapy resistant clones were determined to identify differentially activated kinases with implications in therapy resistance.