Background. Overexpression of oncogenic proteins may be caused by gene amplifications. Cyclin D1 participates in regulation of the cell cycle. Relations between cyclin D1 expression and amplification of CCND1 gene encoding this protein in invasive duct breast carcinomas (IDC) are not fully elucidated. An increased interest is also focused on relations to the estrogen receptor (ER
). Methods and Results.We investigated copy numbers of the CCND1 gene, expression of cyclin D1 and expression of ER
in a group of 60 females and 1 male with IDC. The age range varied from 33 to 89 years (median 57 years). The number of CCND1 gene copies and the number of chromosome 11 was evaluated using FISH, the expression of cyclin D1 and ER
was investigated by IHC. We detected a strong amplification of CCND1 gene (>10 copies per tumor cell nuclei) in 9 patients, weak amplification (<10 copies) in 16 patients. Amplification of the CCND1 gene correlated well with the overexpression of cyclin D1. We observed the overexpression of cyclin D1 also in 13 of 36 patients without the gene amplification; therefore, the mechanism of the protein overexpression is different than that caused by the gene amplification in a proportion of patients. Amplification of the CCND1 gene was associated with a high histologic grade of IDC, whereas cases with cyclin D1 overexpression only were not. 24 of 31 patients with overexpression of cyclin D1 coexpressed ER
. We did not find correlation between expression/amplification of cyclin D1/CCND1 gene and the size of carcinomas and with metastases to the axillary lymph nodes. Conclusions. Amplification of the CCND1 gene is associated with overexpression of cyclin D1 in a majority of IDC. Overexpression of cyclin D1 is related to an increased expression of ER
. Interaction between cyclin D1 and ER
may explain low response to anti-estrogen therapy of some patients.

        Key words: mammary gland, invasive duct carcinoma, CCND1 gene, amplification, cyclin D1, estrogen receptor, FISH.