Considering the multitude of beneficial properties of Sea buckthorn and the limited number of studies investigating the bioactive properties of carotenoids from Sea buckthorn berries in breast cancer, the aim of the present study is to investigate, for the first time, the antiproliferative, antioxidant, and proapoptotic properties of the carotenoids from LSBE on breast cancer cells in two breast cancer cell lines with different phenotypes: T47D (ER+, PR+, HER2−) and BT-549 (ER−, PR−, HER2−). Here, ERBB2 is linked to breast carcinoma.