On this background, we aimed to study the levels of tumor cell proliferation based on Ki67 expression according to specimen type such as whole sections (WS), core needle biopsies (CNB) and tissue microarrays (TMA) from a population-based series of breast cancers, and to study and compare the prognostic value of Ki67 in relation both to specimen type and molecular subgroups of breast cancer. This evidence concerns the gene MKI67 and breast cancer.