The concentration of epidermal growth factor in human and bovine milk was measured by radioreceptor assay. Both human placental plasma membranes and a human epidermoid carcinoma cell were used as the epidermal growth factor receptor source in the assay. The use of placental plasma membrane in the radioreceptor assay gave erroneous results for bovine milk and overestimated the concentration of epidermal growth factor in human milk. Intact cells appear to provide a more accurate measure of the concentration of epidermal growth factor in milk samples. Using A431 cells, we found very low concentrations of epidermal growth factor in bovine milk (less than 2 ng/ml) compared to human milk (30-40 ng/ml). No epidermal growth factor activity was found in several cows' milk-based infant formulas. These results highlight the caution which must be taken when measuring trace substances such as polypeptide growth factors in complex samples such as milk.

Sir,High-risk human papilloma viruses (HPVs) have been consistently identified in 13 -86% of breast tumours in the 10 studies published since 1999 (Lawson et al, 2006). High-risk HPVs of the same type have been identified in both cervical and breast cancer that had occurred in the same women (Hennig et al, 1999;Widschwendter et al, 2004). This observation has lead to the hypothesis that HPVs may be transmitted to the breast during sexual activities (Kan et al, 2005). If this hypothesis is correct, it is likely that HPV-positive breast cancers would occur in women younger than those with HPV-negative breast cancer. This is because HPV genital infections are much more common in young women who have had multiple sexual partners (IARC, 1995).There are only two studies in which the age of women with HPV-positive and -negative breast cancer has been published. There were no differences in the average of age of women with either HPV-positive and -negative breast cancer in a study of Brazilian women (Damin et al, 2004). This is in contrast to a recent study of Greek women in which those with HPV-positive breast cancer were of average age 38 years as compared to average age 53 years for women with HPV-negative breast cancer (P-values for difference ¼ 0.001) (Kroupis et al, 2006).We have reviewed the ages of Australian women with HPVpositive and -negative breast cancer in our study published in this Journal (Kan et al, 2005). These data are shown in Table 1. The average age of women with HPV-positive breast cancer was 55.6 years as compared to 63.8 years for women with HPV-negative breast cancer (P-values for difference ¼ 0.049). These data are compatible with the hypothesis that HPV-positive breast cancers occur in younger women than those with HPV-negative breast cancers, and that high-risk HPVs may have been transmitted by sexual activity with HPV-positive sexual partners. HPV ¼ human papilloma virus. P-value for difference in average ages ¼ 0.049, which is significant at the 95% level. REFERENCES