ABSTRACT
The categories of ER-positive and ER-negative breast cancers are heterogeneous at the molecular level; for example, multiple molecular subtypes of ER-negative breast cancer have been described (1,2,8-11), whose clinical signifi cance remains to be determined. By contrast, ER-positive breast cancers form a continuum, with tumors at one end having a good prognosis and characterized by low histological grade, low proliferation rates, simple karyotypes, low levels of genomic instability, deletions of 16q, and recurrent mutations of PIK3CA. Tumors at the other end of the spectrum have a poor prognosis, are of high histological grade, have complex karyotypes and high levels of genetic instability, and often harbor mutations in key tumor suppressor genes (e.g., TP53) (12-17).
