The aims of doctoral dissertation „Glycosphingolipid expression at breast cancer stem cells after newly synthesized compound 3 ‑ amino ‑ N ‑ (3 ‑ chloro ‑ 2 ‑ methylphenyl) ‑ 5 ‑ oxo ‑5,6,7,8‑tetrahydrothieno[2,3‑b]quinoline‑2‑carboxamide treatment” were to investigate the expression of glycosphingolipids on breast cancer stem cells and non-stem cancer cells after treatment with the newly synthesized compound. Compound activity was analyzed through growth and survival parameters (MTT test), apoptosis, activity (mammosphere formation) and stem cell percentage (CD44+/CD24- and CD133+) and glycophenotype (IV6Neu5Ac- nLc4Cer, GalNAc - GM1b, IV3Neu5Ac-nLc4Cer, GM3, Gb4Cer, GM2, GD3, Gg3Cer and nLc4Cer) on CSC and non-SCC (CD44−/CD24+, CD44+/CD24+, CD44−/CD24− and CD133-) cells of triple negative breast cancer cell lines (MDA-MB-231 and MDA-MB-453). The effect of five different compound concentrations at four-time intervals was analyzed by the MTT assay. All parameters, except the MTT assay, were determined after 48 hours of treatment with a 2 μM concentration of the Compound. The cytotoxic effect, observed in both cell lines, was mediated by apoptosis. The percentage of CSC and the number of mammospheres formed is significantly reduced. CSC and non-SCC differ in glycosphingolipid expression. Glycosphingolipids IV6Neu5Ac-nLc4Cer and GalNAc-GM1b, which have not been previously studied, have been found in both CSC and non-SCC. IV6Neu5Ac-nLc4Cer expression was increased in both subpopulations after Compound treatment, while GM3(Neu5Ac) expression was increased in both subpopulations in MDA-MB-231 cell line. GalNAc-GM1b, Gb4Cer and GM2 were overexpressed on CSC of both cell lines, while GD3 expression was decreased at the CSC in MDA-MB-231 cell line. Due to the effect of the Compound on reducing the percentage of CSC and the formation of mammospheres, the shift of CSC to non-SCC glycophenotype, and taking into account the fact that triple-negative breast cancer is characterized by increased CSC and, consequently, with an increased risk of metastasis, chemoresistance, relapse and mortality, we can conclude that this thieno [2,3-b] pyridine derivative deserves attention as a potentially new drug in the treatment of this type of breast cancer.