Consequently, we were surprised to find no difference in the genomic ERα activity of breast cancer cells grown in obese versus control patient sera, even with the exclusion of patients on aromatase inhibitors at the time of serum collection, suggesting that obesity-related circulating factors promote ERα positive breast cancer cell viability and growth independent of ERα transcriptional activity. The gene discussed is CYP19A1; the disease is obesity due to melanocortin 4 receptor deficiency.