The genes involved in mammary carcinogenesis are not clearly defined, apart from those involved in hereditary breast cancers. Benign breast disease (BBD) and carcinoma in situ(CIS) correspond to some early stages of invasive cancer, according to loss-of-heterozygosity data obtained at different loci in these lesions. The discovery of a second estrogen receptor (ER?), whose gene is located on chromosome 14q22-24, has contributed to the understanding of the mechanism(s) causing mammary cancer. ER? has been proposed as both a good and a bad prognostic factor in breast cancer.