A study with a group consisted of 103 cases of phase I/II invasive breast cancers showed that activated c-jun is expressed predominantly at the invasive front of breast cancer and is associated with proliferation and angiogenesis. Tumor initiation A study was done with liver-specific inactivation of c-jun at different stages of tumor development in mice with chemically induced hepatocellular carcinomas. The result indicates that c-jun is required at the early stage of tumor development, and deletion of c-jun can largely suppress tumor formation. Also, c-jun is required for tumor cell survival between the initiation and progression stages. In contrast to that, inactivation of c-jun in advanced tumors does not impair tumor progression. Breast cancer Overexpression of c-jun in MCF-7 cells can result in overall increased aggressiveness, as shown by increased cellular motility, increased expression of a matrix-degrading enzyme MMP-9, increased in vitro chemoinvasion, and tumor formation in nude mice in the absence of exogenous estrogens. The MCF-7 cells with c-jun overexpression became unresponsive to estrogen and tamoxifen, thus c-jun overexpression is proposed to lead to an estrogen-independent phenotype in breast cancer cells. The observed phenotype for MCF-7 cells with c-jun overexpression is similar to that observed clinically in advanced breast cancer, which had become hormone unresponsive. The invasive phenotype contributed by c-jun overexpression is confirmed in another study. In addition, this study showed increased in vivo liver metastasis by the breast cancer with c-jun overexpression. This finding suggests that c-jun plays a critical role in the metastasis of breast cancer. In mammary tumors, endogenous c-jun was found to play a key role in ErbB2-induced migration and invasion of mammary epithelial cells. Jun transcriptionally activates the promoters of SCF (stem cell factor) and CCL5. The induced SCF and CCL5 expression promotes a self-renewing mammary epithelial population. It suggests that c-jun mediates the expansion of breast cancer stem cells to enhance tumor invasiveness. Vulvar cancer C-jun has been observed overexpressed in Vulvar Squamous Cell Carcinoma samples, in association with hypermethylation-Induced inactivation of the RARB tumor suppressor gene. Indeed, mRNA levels of c-Jun tested higher in Vulvar cancer samples when compared with those of normal skin and preneoplastic vulvar lesions, thus underscoring a cross-link between RARB gene and the oncogene c-Jun. Cellular differentiation Ten undifferentiated and highly aggressive sarcomas showed amplification of the jun gene and JUN overexpression at both RNA and protein levels. Overexpression of c-jun in 3T3-L1 cells (a preadipocytic non-tumoral