Thirty-four cases of ductal carcinoma in situ (DCIS) of the breast, with or without associated benign or invasive disease, were analysed for allelic imbalance (AI) in the region of the BRCA1 gene. AI on 17q12-23 in DCIS was demonstrated in 74% of cases, and in the majority of cases the region of AI included the BRCA1 gene. However, two cases showed AI distal to BRCA1, supporting the presence of a second tumour-suppressor gene on 17q.