The most common malignancy in women is breast cancer. Drug resistance in the treatment of cancer still remains a major clinical concern. Resistance to tamoxifen is seen in half of the recurrences in breast cancer. The antiestrogen tamoxifen gains agonistic property by transactivating ER alpha. PAK1-mediated phosphorylation of serine 305 (S305) of ER alpha leads to resistance to tamoxifen. In our study, PAK1-induced suggestive tamoxifen resistance was designed. According to our hypothesis, phosphorylation of ER alpha-S305 by PAK1 may be reversed by PAK1 transcriptional inhibition by miR-221-3p due to miR-221-3p targeting the 3' UTR of PAK1. For this purpose, we used Real-time PCR (qRT-PCR) to measure the expression level of miR-221-3p in ER-positive breast cancer cell lines (ZR-75-1, MCF7) and breast epithelial cell line, hTERT-HME1, as control in the laboratory incur department. The increase in the expression of PAK1 depending on miR-221-3p may be related to ZR-75-1 cell line which has invasive characteristic but other two ER+ cancer cell lines, MCF7 and HCC1500, have milder cancer severity. miR-221-3p may have a role on regulation of PAK1 expression because miR-221-3p expression level decreases while PAK1 expression level increases in SKBR3 cell line. miR-221-3p and PAK1 expressions in MDA-MB-231 cell line are higher than that of hTERT-HME1 cell line. This may mean that miR-221-3p has no regulatory effect on of PAK1 expression in this cell line. According to these results, miR-221-3p may give crucial information about molecular mechanism of the disease upon PAK1 activity or different mechanisms with respect to histopathology and severity of breast cancer. (C) 2014 Elsevier B.V. All rights reserved.