Tumor Homing Peptides Repository Figure 3. Distribution of tumor homing peptides in TumorHoPe. (A) Average amino acid composition of peptides (SwissProt proteins, tumor homing peptides (THP) and non-cyclic THP), (B) length wise distribution of tumor homing peptides, (C) distribution of peptides based on major sequence motifs, and (D) distribution of peptides based on secondary structure composition. doi:10.1371/journal.pone.0035187.g003 In past, many tumor homing motifs have been identified from homing peptides. These are smallest peptides that can selectively recognize tumor cells and tumor vasculature. One of the common motifs is RGD, which has a tumor homing ability. RGD specifically recognizes integrin (avb3 and avb5) receptor, which is a good marker of angiogenic blood vessels [21]. This motif has been studied extensively and many tumor homing peptides have been designed by incorporating this motif. Similarly, NGR motif has also been identified from tumor homing peptides, which specifically bind to cells expressing aminopeptidase N, a membrane-bound metallopeptidase that plays important roles in tumor angiogenesis [22]. We have examined all such motifs present in tumor homing peptides in our database and computed the number of peptides containing each of the motifs. Distribution of peptides containing most frequent motifs is shown in Figure 3C. It is observed that 45 peptides contain NGR motif, while 36 peptides contain RGD motif. We have also examined other motifs that occur at least in four homing peptides. We have computed secondary structure composition of each peptide and classified peptides based on secondary structure content. As shown in Figure 3D, most of the peptides have no beta-strand or less than 20% beta-strand. A similar trend is observed for helix. Only few peptides have 40–60% helix content. In case of turn, most of the peptides have less than 20% turns content and a sharp decrease in number of peptides having turns up to 60%, is observed. In case of coil, a large number of peptides have most of the residues forming coil. This is expected as it is difficult to maintain a regular structure in small peptides. This is just a primary analysis, and a detailed analysis is required in order to understand the relation between structure and tumor homing capability of the peptides. Discussion TumorHoPe database is developed to serve the scientific community working in the area of peptide based cancer therapeutics. Although many peptide databases, e.g. MHCBN [23], Bcipep [24], HMRbase [25], MimotopeDB [26] , CAMP [27], and ANTIMIC [28] have been published, yet, at present, to the best of our knowledge, there is no database available that provides detailed information about the tumor homing peptides. Therefore, we have developed this database, which not only saves time and effort of researchers involved in the field, but will facilitate the biological discovery process. We have compiled 744 PLoS ONE | www.plosone.org 5 April 2012 | Volume 7 | Issue 4 | e35187