diff --git a/.utilities.R b/.utilities.R
index 4b459fa..ad19221 100644
--- a/.utilities.R
+++ b/.utilities.R
@@ -130,6 +130,25 @@ waitTimer <- function(t, nIntervals = 50) {
 }
 
 
+# ====== PDB ID selection ======================================================
+
+selectPDBrep <- function(n) {
+  # Select n PDB IDs from a list of high-resolution, non-homologous, single
+  # domain, single chain structure files that represent a CATH topology
+  # group.
+  # Parameters   n  num   number of IDs to return.
+  # Value:          char  PDB IDs
+  # Note: the list is loaded from an RData file in the data directory
+
+  load("./data/pdbRep.RData")  # loads pdbRep
+  if (n > length(pdbRep)) {
+    stop(sprintf("You can select no more than %d IDs.", length(pdbRep)))
+  }
+  set.seed(as.numeric(Sys.time()))
+  return(sample(pdbRep, n))
+}
+
+
 # ====== DATA ==================================================================