Objective        To investigate the differential gene expression patterns in human breast cancer MCF-7 cells during the development of tamoxifen (TAM) resistance with cDNA microarray

and determine the effect of S100p on TAM resistance and migration.         Methods        Total RNA was extracted from tamoxifen-sensitive MCF-7 cell line and its resistance cell line MCF-7-TAM resistance (MCF-7R). The probes were hybridized with Agilent gene chips and the fluorescence images of the chips were obtained with Agilent Microarray Scanner. The microarray results were confirmed by real-time quantitative PCR (qRT-PCR) via selecting genes randomly. S100p was down-regulated by siRNA to determine the influence on TAM resistance and migration. Survival analysis was carried out to evaluate clinical relevance of S100p on relapse free survival or distant metastasis free survival by Kaplan-Meier plot.         Results        Of the 4 108 genes with altered expression

there were 1 983 up-regulated genes and 2 025 down-regulated genes in MCF-7R cells compared with MCF-7 cells. The expression levels of many S100 family members were differentially regulated. The expression levels of S100p mRNA and protein were obviously up-regulated in MCF-7R cells

and after RNA interference

MCF-7R cells showed significantly decreased migration and reduced TAM resistance (P0.01). Kaplan-Meier plot indicated that the patients with higher-expressed S100p had lower relapse free survival (RFS

P0.01) and distant metastasis free survival (DMSF

P=0.01) compared with the low-expressed patients (P0.01).         Conclusion        S100p promotes TAM resistance and migration capacity in breast cancer cells.