Ataxia-telangiectasia mutated is a protein kinase that is critical for the maintenance of genomic stability and proper cellular response to redox imbalance. Previous studies indicate that ataxia-telangiectasia mutated is also required for expression of the insulin-like growth factor-1 receptor in human fibroblasts. As insulin-like growth factor-1 receptor is critical in mammary gland development, we sought to determine the relationship between ataxia-telangiectasia mutated and insulin-like growth factor-1 receptor using mouse as a model system. Knockdown of ataxia-telangiectasia mutated in cultured mouse mammary epithelial cells resulted in a significant reduction in insulin-like growth factor-1 receptor mRNA levels. Using a conditional knockout mouse model, we observed that loss of ataxia-telangiectasia mutated resulted in a severe decrease in both insulin-like growth factor-1 receptor mRNA and protein expression. These results support the observation that insulin-like growth factor-1 receptor expression is ataxia-telangiectasia mutated -dependent in the mammary epithelium, and given the critical role that insulin-like growth factor-1 receptor plays in mammary gland development, suggests that ataxia-telangiectasia mutated may be critical in the development of this organ as well.