['Regulation of physiological processes by glucocorticoids is achieved by binding to the glucocorticoid receptor (GR) and subsequent modulation of gene expression, either by DNA binding-dependent mechanisms or via protein-protein interaction with other transcription factors. The purpose of this study was to define the molecular mechanism of GR underlying the control of mammary gland development and lactation.', 'To dissect the mechanism of GR action in the mammary gland, we used genetically modified mice carrying a DNA binding-defective GR. These mice retain the ability to regulate transcription by protein-protein interaction but fail to control gene expression by DNA binding-dependent mechanisms. Thus, they allow the study of the mode of GR action in vivo.', 'The development of the mammary gland and milk protein synthesis during lactation were studied using histological and biochemical methods.', 'Our findings demonstrated that the lack of the DNA binding function of GR impairs the ductal development of the mammary gland in virgin females and that this can presumably be accounted for by reduced proliferation of epithelial cells. In contrast, lactating females have normally differentiated mammary glands and are fully capable of milk protein production. This is in good agreement with the demonstration that the DNA binding-defective GR is still able to interact with phosphorylated Stat5 proteins, suggesting that transcriptional regulation by protein-protein interaction forms the basis of glucocorticoid action in this process.'] Are mammary gland development and lactation controlled by different glucocorticoid receptor activities?