ANPEP and squamous cell carcinoma: Included variables: CD13 expression in stromal cells (negative expression (ref.)vs. positive expression), CD13 expression in NSCLC tumor cells (negative expression (ref.)vs. positive expression), tumor stage (stage I (ref.)vs. stage II vs. stage III), lymph node status (pN0 (ref.)vs. pN1 vs. pN2 vs. pN2), and tumor histology (squamous cell carcinoma vs. adenocarcinoma vs. large cell carcinoma).