2.4. PPI Network Construction and Hub Gene Identification

We used the Search Tool for the Retrieval of Interacting Genes (STRING) database (http://string-db.org/) to analyze protein-protein interaction (PPI) networks. It was essential to display the molecular mechanisms of key activities in endometriosis. To investigate the potential PPI relationships, the previously identified DETFs and DEIRGs were mapped to the STRING database. The PPIs were extracted with a combined score > 0.9. Subsequently, the visualized PPI network was constructed by Cytoscape software (version 3.7.1). The Hmisc R package (version 4.4.2) (https://hbiostat.org/R/doc/sintro.pdf) was utilized to test the correlations between DETFs and DEIRGs with the cutoff criteria set as correlation coefficient > 0.5 and P < 0.001. The Molecular Complex Detection (MCODE) plugin of Cytoscape software was utilized to recognize the most prominent clustering modules. Functional enrichment analysis of the genes in individual modules was achieved by DAVID, an online tool (https://david.ncifcrf.gov/), with a significance threshold of P < 0.05. Nodes with higher connectivity tend to be more important for maintaining the stability of the entire network. Therefore, cytoHubba, a plugin in Cytoscape, was used to screen out hub genes.