Background and Aim: The mutation in the wild-type tumor suppressor gene p53 is the most common genetic change in human tumors. In addition, the normal function of p21, which is both antiproliferative and an inhibitor of the cell cycle, is disrupted in some types of cancer. Meanwhile, cyclin D1 is a member of the cyclin protein family that is involved in regulating cell cycle progression. This study aimed to assess the expressions of the cell cycle inhibitory proteins p21, cyclin D1, and tumor suppressor gene p53, as well as their influence on the expressed histopathological changes in breast cancer tissues.

Materials and Methods: Overall, 40 breast tissue specimens were investigated in this study, 30 of which were cancerous, while 10 were healthy tissues. p53, p21, and cyclin D1 expression patterns were detected using an immunohistochemistry (IHC) system.

Results: The IHC reactions for p53 were positively observed in 27/30 (90%) cancerous tissues, compared with 2/10 (20%) normal breast tissues. For p21, reactions were observed in 28/30 (93.33%) cancerous tissues and 3/10 (30%) control tissues. For cyclin D1, reactions were observed in 25/30 (83.33%) cancerous tissues and 1/10 (10%) control tissues. The differences between the breast cancer tissues and the control tissues were statistically highly significant (p<0.01).

Conclusion: The high expression rates of p21, cyclin D1, and p53 in malignant breast cancer cells with little or no regulatory role might imply mutational events in these proteins operating in concert with a variety of other genetic mutations in these tissues, which may play a molecular role in the development and/or progression of breast carcinogenesis.

Keywords: breast cancer, D1 protein, Iraq, P21, P53.