Based on correlative analysis of Id1 expression during mammary developmentand experimentationwithcelllines, Id1 has been proposed to regulate mammary PD325901 differentiation and cell fate decisions. Using a highly sensitive and specific antibody we now provide definitive evidence that Id1 is not abundantly expressed in the mammary epithelium. Previous studies have reported Id1 expression in the mammary gland based on immuno-staining with a polyclonal antibody or by northern blotting of whole mammary extracts. We show that the polyclonal antibody has poor sensitivity and low specificity, and returns strongly positive immunostaining in both wildtype and Id1-null mammary glands. We detect Id1 expression in a number of stromal cell types, thus northern blotting of mammary extracts most likely detects Id1 expression in stromal cells rather than in the epithelium. However, Id1 may be expressed in rare epithelial cells within the mammary gland and we are currently investigating this possibility. Id1 expression has previously been reported to correlate with poor prognosis in breast cancer, however that study used the polyclonal antibody that we report here to be non-specific and insensitive in mouse tissues. While we did 
not readily detect Id1 in the normal mammary epithelium, we did detect Id1 expression in a mouse mammary cancer model, and have similarly detected Id1 in human breast cancer cell lines and clinical cases. This suggests that Id1 expression is activated during mammary neoplasia and that the prognostic significance of Id1 expression in breast cancer cohorts should be re-evaluated using this new monoclonal antibody, which we are currently pursuing. Based on previous reports, we predicted that overexpression of Id1 in the luminal epithelial cells of the mammary gland would dramatically alter mammary development and pregnancy-related maturation. However, we demonstrate that Id1 expression alone is not sufficient to alter luminal epithelial cell fate nor to prevent terminal differentiation. Id1 transgenic mice underwent normal pubertal and pregnant mammary gland development, and were able to lactate and feed pups as normal. These data raise the question of why Id1 failed to regulate differentiation or mammary development. Unlike cells from control mice, cells taken from TRE-Id1 + MTB bi-transgenic mice were fully transformed by transduction with oncogenic h-RasV12 expression as previously reported, demonstrating that the Id1 transgene is active in these cells. The failure to regulate mammary development may therefore be a result of expression of the transgene in a non-physiologically relevant cell type, as we do not currently know whether the MMTV promoter directs transgene expression in the appropriate cell type in which Id1 is physiologically expressed. These results are consistent with a recent report that failed to detect a histological phenotype following Id1 transgene overexpression in the prostatic epithelium. The use of virtual screening to discover new inhibitors is becoming a common practice in modern drug discovery. Receptor-based virtual screens seek to “dock” members of a chemical library against a given protein structure, predicting the conformation and binding affinity of the small molecules. A large number of programs are available for this purpose, such as DOCK, FlexX, GOLD, and AutoDock. This study focuses on AutoDock 4 and AutoDock Vina, both notable for being among the few docking programs that are freely available for academic and Nilotinib industrial use.