In situ duct carcinoma (DCIS) is a heterogeneous group of lesions which has recently been subdivided into three types: well-differentiated (type I), intermediately differentiated (type II) and poorly differentiated (type III) DCIS. Fourteen cases of DCIS and 11 of DCIS with minimal invasion were analysed for mRNA levels of beta-actin, EGFR, c-cerbB2, MTS1, k-ras, RB, BRCA1, cyclin E, and c-myc genes. A microdissection technique was used on paraffin-embedded tissue. A statistically significantly higher expression of cyclin E oncogene and MTS1 tumor suppressor gene was seen in type III DCIS than in the other types, while no significant differences in the mRNA expression patterns of the other genes were observed. These data are consistent with the fact that poorly differentiated DCIS is a readily recognizable class of tumours that have a particularly aggressive behaviour and probably unique histogenesis.