--- a
+++ b/R/PPI.R
@@ -0,0 +1,23 @@
+#' @title Defining protein-protein interactions (PPI) over a list of genes,
+#' @description This function uses STRING-api. The outcome of STRING analysis
+#'   will be stored in comma-separated values files.
+#' @export
+#' @param data A gene list.
+#' @param FileName A string vector showing the name to be used to save the
+#'   resulted table. If null, no file will be exported
+#' @param species The taxonomy name/id. Default is "9606" for Homo sapiens.
+#' @importFrom httr content
+#' @importFrom utils read.table write.table
+#' @return Either CSV files stored in the user's file system and its
+#' corresponding `data.frame` object in R or and R object containing that
+#' information.
+PPI <- function(data, FileName = NULL, species = "9606") {
+  repos <- retrieveURL(data, species, "tsv")
+  # Process API request content
+  repo_content <- content(repos)
+  results <- data.frame(repo_content[, , 1])
+  if (!is.null(FileName)) {
+    write.csv(results, file = paste0("PPI-", FileName, ".csv"))
+  }
+  return(results)
+}