Progression through G1 phase of the eukaryotic cell cycle is tightly controlled by cyclin-dependent kinases (CDK). These proteins form part of a regulatory pathway including the cyclin-dependent kinase inhibitor (CKI) p16, D-type cyclins and the product of the retinoblastoma gene pRb. Aberration of any one of these components may lead to uncontrolled proliferation contributing to neoplasia. Three of these proteins, cyclin D1, pRb and p16, were analysed by immunohistochemistry on archival paraffin sections to determine whether expression patterns were different in preinvasive ductal carcinoma in situ (DCIS) and invasive breast tumours relative to normal. Genetic analysis of the gene encoding cyclin D1 (CCND1) was also carried out, using an intragenic restriction fragment-length polymorphism (RFLP) to assess possible allelic imbalance. A majority of the tumours studied (approximately 90%) showed abnormalities in expression of at least one of these proteins. Overexpression of cyclin D1 was found in approximately 49% cases, reduced expression of p16 in approximately 46% and reduced expression of pRb in approximately 37%. Allelic imbalance of cyclin D1 was found in approximately 57% cases.