Among the investigated breast cancer cell types (ER-positive: three variants of MCF-7 cells, T-47-D cells; ER-negative: MDA-MB-231 cells and the triple-negative HCC1806 and HCC1937 cells), NPY receptors were only detected in ER-positive cells (Fig. 3 and 7) and identified as the Y1R subtype by confocal microscopy (Fig. 4) and radioligand binding (Fig. 3 and 7). This evidence concerns the gene ESR1 and breast cancer.