The determination of the constitutive expression of mRNA was carried out by RT-PCR of the long isoform of the leptin receptor (ObRb), homologous region for any of the six different isoforms of the leptin receptor (ObRt), estrogen receptor (ER) and aromatase (CYP19A1) in the three breast cancer cell lines prior to stimulation with leptin and tamoxifen, as shown in Fig. 1. Here, ESR1 is linked to breast cancer.