Ductal carcinoma in situ (DCIS) is considered to be a non-obligate precursor of invasive breast cancer (IBC). Use of screening mammography has led to a substantial increase in detection of DCIS over the past 2-3 decades. About 5-14% of patients diagnosed with DCIS and treated with breast-conserving therapy, with or without radiation, develop an ipsilateral IBC and 1-6% develop a contralateral IBC over a period of 10 years. However, natural history studies have shown that, in the absence of treatment, 14-53% of DCIS cases develop IBC if followed for up to ~30 years. Treatment of DCIS is variable, and many DCIS patients are either under- or over-treated. Elucidation of the molecular changes detectable in DCIS lesions that are associated with risk of IBC development is critically needed, as this may help not only to reduce risk of development of IBC but also to prevent overtreatment of patients with lower risk of IBC. In this regard, a multigene expression assay, consisting of genes related to proliferation, as well as PR and GSTM1, was recently shown to predict risk of subsequent ipsilateral IBC in women with DCIS. Similarly, immunohistochemically-detected expression of p16, COX-2, and Ki67 has also been associated with increased risk of IBC development. However, these findings require confirmation. Furthermore, novel prognostic (and ultimately predictive) markers may emerge from assessment of gene expression patterns on a global scale. In this regard, microRNAs (miRNAs), which are noncoding RNAs that are master regulators of gene expression, are thought to contribute to the development of invasive cancer. Against this background, our overarching goal is to facilitate early detection of patients with DCIS at risk of IBC development. To this end, building upon our previous work, we propose to use clinical data and archived formalin-fixed paraffin-embedded (FFPE) tissue from a large, population-based multi-center cohort of 7,275 patients initially diagnosed with DCIS in community-based health plans and followed for subsequent IBC development, to identify and then validate miRNA expression changes associated with risk of subsequent IBC, to evaluate risk of IBC in association with 2 previously identified sets of markers (Oncotype DX DCIS score; positivity for p16, COX-2, and Ki67 protein expression), and to examine the association between clinical factors and risk of subsequent IBC in the largest such study to date. Our molecular epidemiologic study, which proposes to apply state-of-the art technologies to archived DCIS FFPE specimens for the detection of molecular changes associated with risk of IBC development in a large, multi-center population-based cohort of women initially diagnosed with DCIS, has the potential to lead to approaches that will help to refine identification of women who need enhanced surveillance and early aggressive treatment.