To further confirm the selectivity of ZnPc-GnRH conjugate towards breast cancer cells is mediated by the GnRH surface receptor, MCF-7 cells were pre-treated with a GnRH antagonist Cetrorelix (at 7.5×10−7 M) for 12 hr, followed by the monitoring cellular uptake of ZnPc-GnRH (at 5.0×10−6 M) for 2 hr. This evidence concerns the gene GNRH1 and breast cancer.