The multistep model of carcinogenesis in the breast suggests a transition f
rom normal epithelium to invasive carcinoma via non-atypical and atypical h
yperplasia and in situ carcinoma, Within the breast, these proliferations a
re heterogeneous in their cytological and architectural characteristics. Th
is review considers the evidence supporting a precursor role for these prei
nvasive lesions, Copyright (C) 1999 John Wiley & Sons, Ltd.