A panel of 36 cases of preinvasive breast lesions, including 35 cases of ductal carcinoma in situ (DCIS), has been examined for mutation of TP53, allelic imbalance (AI) on 17p13, and expression of TP53, in a number of cases, has been studied using immunohistochemistry. Areas of DCIS, with or without adjacent invasive or benign cells, have been separately microdissected from paraffin-embedded sections and analysed by PCR for genetic changes to chromosome 17p13. TP53 mutations and AI on 17p have been identified in cases of 'pure' DCIS as well as those with associated invasive carcinoma and, furthermore, have been identified in well-differentiated lesions as well as poorly differentiated ones.