ESR1 and breast cancer: Based on these promising findings, the growth inhibitory effects of conjugate 10 and the related 2ΔHis-3d-Tic (3, 5 and 12) conjugates, as well as the 10ΔGly-NH-Et (7 and 14) containing compounds, were studied on estrogen receptor negative (ERα-) MDA-MB-231 breast cancer cells.