show Abstracthide AbstractIn current study, we performed 12-weeks time-course mRNA expression analysis on the biological sextuplicate samples of estrogen receptor (ER)-positive MCF-7 breast cancer cells in presence or absence of tamoxifen to capture cellular state changes associated with acquisition of tamoxifen resistance. mRNA (1 mg) obtained from the MCF-7 cells was used for Poly A+ mRNA-sequence using the Illumina TruSeq RNA Library Prep Kit v2 according to the manufacturer protocol. 100 base pair-end reads or 36 base single-end-reads were obtained using Hiseq2500 (Illumina) and analyzed by analysis software provided by Illumina. To ensure the validity of the experiment, expression of representative genes (such as EGFR, ErbB2 (HER2), IGF-IR, NCOA3 (AIB1), MYC, CCND1 (cyclin D1) and CCNE1) known for tamoxifen resistance in vitro and clinical setting (Musgrove 2009) were confirmed.