Cancer is characterized by uncontrolled growth (proliferation) of cells [
1‐
3] and their ability to invade other organs or tissues, either by invasion or metastasis [
2‐
4]. Although there are many types of cancer, all begin with uncontrolled growth of abnormal single cells in the body [
3]. Breast cancer is the most common malignant neoplasm in women. Breast cancer is the commonest cancer among women in United Kingdom [
5]. Breast cancer comprised 30.4 % of all female cancers in Malaysia in 2002 [
6]. Approximately 182,000 estimated new cases of women with breast cancer are seen in the United States each year [
7,
8]. The number of women who die of breast cancer is approximately 46,000 each year in USA [
7]. One out of eight women in United States and out of ten women in Europe is at risk of developing breast cancer at some point during her lifetime [
9]. Most deaths of women with breast cancer arise not as a result of primary tumor but from its metastatic spread to distant sites in the body [
10,
11]. Once spread and secondary masses are formed, breast cancers are usually incurable [
12]. Approximately one-third of the node negative breast cancer patients develop metastatic disease, whereas the other two-thirds do not develop metastasis despite not receiving chemotherapy [
13]. Breast cancer is considered to be a systemic disease as most breast carcinoma metastasizes before diagnosis of the primary lesion [
14]. Therefore, early detection of metastasized lesion and identification of more effective therapeutic modalities for metastatic disease are necessary if the prognosis for patients with advanced breast cancer is to improve. nm23 gene located on chromosome 17 q21.3 (A subunit by nm23-H1 and B subunit by nm23-H2) encodes nucleoside diphosphate kinase. It has been reported that differential regulation of nm23 by p53 in different cell types is an important component in the molecular mechanisms of tumor metastasis [
15,
16]. Other study reported that high nm23 expression is associated with older age (> 35 years) and smaller tumor size [
17]. Also high nm23 expression is associated with the absence of distant metastasis [
18]. Low expression is predictive of distant metastasis and appears to be a risk factor that is independent of the presence or absence of positive axillary nodes at diagnosis [
18]. In contrast, several studies of lung and pancreatic carcinoma reported that nm23 expression correlated with advanced tumours [
19‐
22]. Thus, the putative usefulness of nm23 in prediction of the clinical course of breast cancer patients remains to be identified. The main purpose of the study was to find the expression pattern of nm23 proteins in different types of breast cancer with or without lymph node involvement and to determine its role in breast cancer using tissue microarray.