It has, therefore, been hypothesized that tamoxifen treatment of breast cancer may be inefficient in tumour cases that harbour genetic lesions which result in elevated levels of cyclin D1 or A, or that lead to reduced levels of cdk inhibitors p21 or p27 (Michalides, 1999; Cariou et al, 2000). This evidence concerns the gene CCND1 and breast carcinoma.