Both estrogen receptor/progesterone receptor+ (ER/PR+) cells [79] and ER/PR- human BC cell lines, which have been selected to preferentially metastasize to bone [82], are inhibited from proliferating on stromal fibroblast monolayers and form dormant single or oligocellular clones when incubated at clonogenic density. This evidence concerns the gene ESR1 and breast cancer.