To investigate the effect of ATR deficiency in human breast fibroblasts on tumor growth in vivo, 10 nude mice were randomized into two groups (n=5) and orthotopic breast tumor xenografts were created under the nipples by co-implantation of MDA-MB-231 cells (2×106) with either N64-sh or N64C cells (2×106). Here, ATR is linked to breast neoplasm.