The advantages of APN as a biomarker for tumor targeting include the following: (1) although APN has been found to express in both normal and angiogenic endothelial cells, it has been suggested that the tumor-targeting property of NGR-drug conjugates only recognizes the APN expressed in angiogenic vessels but not in normal ones due to their different post-translations [31, 32]; (2) it has been demonstrated that APN is overexpressed in a number of tumor cells and plays a crucial modulator role in tumor angiogenesis, metastasis, and survival [16, 28]. This evidence concerns the gene RTN4R and neoplasm.