2.3 Data processing
The raw data were prepared using the ‘affy’ package in the Bioconductor project of the R software to obtain the expression values of the miRNA and mRNA probe set, including background correction, normalization, and summarization.10-12. Then, the self-written Perl language script program was used to convert the expression value of the mRNA probe set into the expression value of the gene (expressed by NCBI Entrez Gene ID, http://www.ncbi.nlm.nih.gov/gene). Finally, array were standardized, that is, the median expression value of all genes was subtracted from the expression value of each gene for each sample, so that the normalized miRNA and gene expression values were obtained. Only genes exhibiting an expression level fold change (FC) >1.5, or 1/FC<1.5 in the BPD group compared with that in the control group, were considered to reveal altered expression levels and were selected for further analysis.