Because mammographic density is associated with epithelial and stromal proliferation [8,9], and circulating IGF1 and IGF binding protein (IGFBP)-3 are associated with premenopausal breast cancer in some studies [10,11], but not all [12-14], the IGF pathway is a compelling candidate for examination with respect to mammographic density. This evidence concerns the gene IGFBP3 and breast cancer.