A correlation between the incidence of breast cancer and an observed change in the X-ray diffraction pattern of hair from the afflicted individuals was first reported in 1999. Since that time, over 500 hair samples have been analyzed in double-blinded breast cancer studies with no false negatives being detected. To correlate this observed change with the presence of breast cancer, we examined whiskers removed from nude mice prior to and 8 weeks after subcutaneous implantation of a human breast cancer cell line. Here we show that the change observed in human hair was also evident in whiskers and that it appeared soon after cancer cell implantation. (c) 2005 Wiley-Liss, Inc.