Since we found that the estrogen receptor breast cancer cell line MCF7 was the most permissive to virus replication, it would be possible to imagine a scenario in which estrogen receptor positive breast tumors may act as a sort of SARS-CoV-2 reservoir that can continuously supplies new viral particles thus exacerbating COVID-19 aggressiveness and the consequences associated to this disease. This evidence concerns the gene ESR1 and breast cancer.