IGF1R and breast cancer: Several lines of evidence suggest that the IGFIR is involved in breast cancer progression: breast cancer cell lines express the IGFIR [6]; transgenic mice over-expressing a constitutively active IGFIR develop mammary tumors at an early age [7]; transgenic mice expressing a truncated IGF-I, with decreased affinity for IGF binding proteins, have increased incidence of spontaneous mammary tumors, and tumors appear earlier when combined with p53 mutation [8,9].