MicroRNAs (miRNA) are approximately 22-nucleotide noncoding RNAs that negatively 
regulate protein-coding gene expression in a sequence-specific manner via 
translational inhibition or mRNA degradation. Our recent studies showed that 
miRNAs exhibit genomic alterations at a high frequency and their expression is 
remarkably deregulated in ovarian cancer, strongly suggesting that miRNAs are 
involved in the initiation and progression of this disease. In the present 
study, we performed miRNA microarray to identify the miRNAs associated with 
chemotherapy response in ovarian cancer and found that let-7i expression was 
significantly reduced in chemotherapy-resistant patients (n = 69, P = 0.003). 
This result was further validated by stem-loop real-time reverse 
transcription-PCR (n = 62, P = 0.015). Both loss-of-function (by synthetic 
let-7i inhibitor) and gain-of-function (by retroviral overexpression of let-7i) 
studies showed that reduced let-7i expression significantly increased the 
resistance of ovarian and breast cancer cells to the chemotherapy drug, 
cis-platinum. Finally, using miRNA microarray, we found that decreased let-7i 
expression was significantly associated with the shorter progression-free 
survival of patients with late-stage ovarian cancer (n = 72, P = 0.042). This 
finding was further validated in the same sample set by stem-loop real-time 
reverse transcription-PCR (n = 62, P = 0.001) and in an independent sample set 
by in situ hybridization (n = 53, P = 0.049). Taken together, our results 
strongly suggest that let-7i might be used as a therapeutic target to modulate 
platinum-based chemotherapy and as a biomarker to predict chemotherapy response 
and survival in patients with ovarian cancer.